Recent Features
| |||||||||||||
Ext4 filesystem hits Android, no need to fear data loss (ars technica)Ext4 filesystem hits Android, no need to fear data loss (ars technica)Posted Dec 28, 2010 3:38 UTC (Tue) by Nick (guest, #15060)In reply to: Ext4 filesystem hits Android, no need to fear data loss (ars technica) by rich0 Parent article: Ext4 filesystem hits Android, no need to fear data loss (ars technica)
What's difficult about fsync?
If you have a volatile writeback cache, the app needs to specify at what point it expects the data to be durable. If every other function requires data to be durable, then every other function needs to call fsync. If you don't want that, then you can't use a writeback cache.
Unrelated files will not be corrupted or lost if fsync is not used (it's a perfectly valid mode to leave maximum amount of dirty data in writeback cache).
If the disk is not bandwidth constrained, it will write back data older than a particular time. For guarantees though, fsync should be used. There wouldn't be much problem in fsyncing every 2 minutes of video to ensure no more than that is lost after power failure.
(Log in to post comments)
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 28, 2010 13:42 UTC (Tue) by butlerm (subscriber, #13312) [Link] What's difficult about fsync?The problem is that fsync is a much slower, more heavyweight operation than is actually needed in most cases. Usually what you need on recovery is consistency, not durability. Rename-replace points are an unusually convenient place to provide consistency. The FS just needs to go a little out of its way to make sure that upon recovery after a rename replace you either get the old version or the new version of the file being replaced To be sure, the quick and dirty way of providing consistent rename replace is for the FS to force the new version to permanent storage before committing the rename transaction. That is not really necessary though. You can also create rename undo records and on recovery roll back to the last completely written version. No sync serialization required, let alone the user visible thumb twiddling variety.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 28, 2010 14:01 UTC (Tue) by mjg59 (subscriber, #23239) [Link]
"Unrelated files will not be corrupted or lost if fsync is not used"
That's true, but the fact that it's POSIXly ok to transform the "Write to temporary file, rename over original" sequence into "rename over original, then commit new data to disk" means that the definition of "unrelated" is not always what application developers expect. The question is whether it's expected that Linux filesystems give stronger guarantees than POSIX, and I think the outcome of previous discussions means that (at least as far as general purpose filesystems go) the answer is yes.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 28, 2010 17:59 UTC (Tue) by iabervon (subscriber, #722) [Link]
POSIX doesn't make any guarantees at all about what happens when the system stops and starts again, so it would be hard for Linux to be useful and not make stronger guarantees. Or, rather, Linux filesystems don't guarantee anything more (the reason your system crashes could conceivably be that your video driver has started sending data to your hard drive instead of your video card, entirely wiping out your storage), but the behavior that Linux rarely diverges from is substantially better than POSIX requires.
fsync() is kind of weird in that, if the system never crashes, it has no visible effect; and if the system ever crashes, it might not come back at all or it might do something random that happens to undo all the effects of fsync().
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 28, 2010 19:46 UTC (Tue) by Trelane (subscriber, #56877) [Link]
So it sounds like fsync would work except that it flushes data and metadata, thereby causing performance issues, yes? If so, a nice solution may be to introduce a metadata-only sync?
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 28, 2010 20:46 UTC (Tue) by quotemstr (subscriber, #45331) [Link] *sigh* Not this argument again. Go reread the old threads: LWN ran carefully balanced coverage of the Great FSync War.To respond to your particular comment, the issue is really that we need a write barrier, while fsync specifies a full flush, a much stonger and unnecessary operation.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 28, 2010 22:55 UTC (Tue) by neilbrown (subscriber, #359) [Link]
This observation is somewhat amusing in light of the fact that barriers in the Linux FS/Block layer were recently replaced with controlled flushing.
fsync isn't a "full flush" (except in ext3), it is a controlled flush of a single file. "sync" is "full flush".
What you really need is dependencies. Journalling filesystems use dependencies a lot to make sure that things get written in the "right" order. They submit lots of antecedent writes, then a flush, then the dependent writes. e.g. metadata-to-journal, journal-commit-block, metadata-to-filesystem.
If you really wanted to export journalling-style protection to user-space apps, I would look at allowing dependencies to be specified, either implicitly (close -> rename) or explicitly.
Despite comments above and elsewhere, I don't think write/close/rename is guaranteed to provide atomicity in Linux. Ted Ts'o recently wrote:
> The implementors of a number of mainstream file systems (i.e., ext4 btrfs, XFS) have agreed to do the equivalent of #1 (i.e., initiating writeback, but not necessarily waiting for the writeback to complete) in the case of a rename that replaces an existing file.
As there is no wait or dependency, there is no guarantee (as I read it).
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 29, 2010 2:09 UTC (Wed) by quotemstr (subscriber, #45331) [Link] This observation is somewhat amusing in light of the fact that barriers in the Linux FS/Block layer were recently replaced with controlled flushing.And the second part of that plan is to use threads and internal queues to achieve the same overall effect; that's not really an option for userspace. As for citing Tso's: his views on the subject are widely-known; that doesn't make them correct. There is nothing wrong with an application relying on transactional rename semantics, and asking userspace to not use reasonable, convenient behavior that's worked for years is simply not a tenable position.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 29, 2010 3:01 UTC (Wed) by neilbrown (subscriber, #359) [Link]
1/ I wasn't citing Ts'o's opinion. I was citing his report on a meeting of several filesystem developers. He wasn't say what "should". He was saying what "is".
2/ The transaction rename semantics that ext3 provided were really a bug (or mis-feature or short-sighted design or whatever). I think it was only ext3 that provided it, certainly not ext2, probably not XFS, not sure about reiserfs. Any userspace that relies on it (without checking that the filesystem in use is ext3) is buggy.
That said - I think that it would be good if Linux did provide some transactional guarantees that didn't require fsync and that user-space could rely on across all filesystems. I suspect that enhancing rename would be a good start and could be done with little or no performance cost. It might even be good to provide something more explicit. But I don't think that 'barriers' is the right model.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 29, 2010 4:45 UTC (Wed) by iabervon (subscriber, #722) [Link]
The thing is that transactional rename semantics works for all POSIX-compliant filesystems when your computer doesn't crash. Anything that's only needed if your computer crashes isn't going to be used consistently, because developers' computers crash so rarely and it's awkward to include in an application regression test suite.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 29, 2010 8:59 UTC (Wed) by neilbrown (subscriber, #359) [Link]
fsync is only needed if your computer crashes. Not using it works perfectly when your computer doesn't crash.
By your argument, fsync will not be used consistently, and that appears to be true to some extent.
The implied conclusion seems to be that all filesystems should be mounted "-o sync" so that fsync is not needed. Strangely, I have not heard that conclusion being proposed explicitly....
I certainly agree that interfaces should be hard to misuse (the Rusty principle) but that must be balanced against the dictum against making things fool proof as then only a fool would use them. In this case, while it is quite possible to misuse fsync (e.g. by not using it), it really is an appropriate interface.
And while regression tests suites are incredibly valuable and there should be more of them etc etc, one hopes that developers don't depend of them as the sole means of ensuring correctness, but also read documentation, try to understand the systems they work with, and write code accordingly. (one also hopes for a pony.... but no, only a piece of coal in my stocking again)
Syncing has significant performance penalties. Posted Dec 29, 2010 14:26 UTC (Wed) by gmatht (guest, #58961) [Link]
The applications that actually really need durability of data written in the last few seconds don't seem to be that common... one might hope that the people who wrote the database your bank uses, would know about fsync. Even on ext4 a trivial fsync takes 50ms[1], which is forever in computer times, much more even that the ~0.1ms for creating new files, but losing everything because the filesystem decided to write out the metadata before the data (violating atomicity) isn't that great either.
Apparently xsyncfs provides a system that is synchronous from the users point-of-view with an overhead of only 3%-7%. That might be an acceptable default. Xsyncfs doesn't seem very well documented, but is just one avenue of providing good reliability properties without the massive performance penalties from fsync/-o sync.
[1] www.ucc.asn.au/~mccabedj/fsync_benchmark.c
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 29, 2010 17:42 UTC (Wed) by iabervon (subscriber, #722) [Link]
Using fsync only really makes sense if you're trying to get stuff written to disk before sending a message out of the system; otherwise, it won't be possible to tell whether the fsync didn't actually do anything or the system crashed before it returned. So you need to use fsync after writing a received email message to disk and before telling the remote server that you've got it.
I believe that the model that most people have of filesystems is that what's recovered after a system crash is like a snapshot of the filesystem that you would see in a running system if you were taking the snapshot with ordinary system calls and could therefore see all the race conditions you can see between programs; however, there is arbitrary random damage because the system crashed, and the latest snapshot may not be particularly recent.
With this model, fsync is easy to (know to) use in cases where you want to make sure that the snapshot is sufficiently recent, but not for cases where it is necessary to avoid the recovered state being something that couldn't have been a snapshot.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 30, 2010 0:14 UTC (Thu) by neilbrown (subscriber, #359) [Link]
> I believe that the model that most people have of filesystems is that ...
That is an overly naive model of a filesystem. It assumes almost completely linearisation of operations on their way to storage. Any re-ordering in the page cache before writeback or in the device queue via an IO scheduler will invalidate that model, and as you can imagine such re-ordering happens a lot.
The correct model is "nothing is safe until you call sync or fsync or some other variant", with the understanding that 'sync' is effectively called every 30 seconds or so.
I'm glad it is obvious that you need to call fsync (on both the file and the directory you created the file in) before acknowledging the receipt of a file (e.g. an email) over a network connection.
However exactly the same is true when moving a file by copying it. If you copy a file (possibly transforming it on the way) and the remove the original you really must fsync the new copy before unlinking the old. You should also fsync the directory, though if you rename the new (after fsyncing it) to replace the old, then the fsync of the directory is not required.
Note that "mv" doesn't do the fsync when moving a file between filesystems (which requires a copy/unlink). So if you use mv and then crash you could quite possibly lose both copies. And mv doesn't even have an option to request the fsync.
Now you might suggest that this should "just work" without mv needing to call fsync. But I think you would find it quite difficult to design the filesystem semantics that would allow this to always be safe, especially as you need interaction between two separate filesystems (unlink in one must not commit until writes in the other have committed). ... other than mounting everything with '-o sync' of course.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 30, 2010 1:33 UTC (Thu) by iabervon (subscriber, #722) [Link]
My model can't really fail to be accurate, since it includes the possibility of arbitrary deviations from the predicted outcome. And, actually, nothing is safe at all; your storage medium might fail, your video driver might scribble over your disk or your dirty pages, your hard drive might read garbage out of memory losing power and write it with the power left in its capacitors. I actually suspect that, based on the model I stated, a more common and more extensive source of differences from some potential snapshot is things that syncing couldn't have helped with than things that syncing could have helped with (with the exception of ext4 having a particularly common and obvious divergence).
There's also been not that long in the UNIX tradition when you could be reasonably confidant that a power failure shortly after you changed something in a directory wouldn't trash other things in the directory, making it kind of irrelevant whether you'd called fsync on the directory to make sure that the disk was correct before it got corrupted.
In general, there's a tradeoff among filesystem complexity, slowness, and
(In fact, my model does require an fsync when moving a file by copying it, at least across directories; the snapshotting process could read the destination directory before you write the file and the source directory after you unlink it.)
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Jan 1, 2011 18:54 UTC (Sat) by butlerm (subscriber, #13312) [Link] If you really wanted to export journalling-style protection to user-space apps, I would look at allowing dependencies to be specified, either implicitly (close -> rename) or explicitly.User specified dependencies are completely unnecessary. All that is necessary is for filesystems to exercise a little bit of additional effort to preserve POSIX semantics across system crashes. That means atomic renames, among other things. Very simple: log the rename in the journal. Keep the old inode around until the data associated with the replacement inode commits to disk. If the system crashes in the meantime, on recovery undo the rename, thereby restoring the association between the name and the old inode. No need to commit the new inode data implicitly or explicitly prior to writing any metadata journal entries. No need to call fsync unless you actually want synchronous behavior. No need to start writeout on the new inode immediately, either.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Dec 28, 2010 21:44 UTC (Tue) by iabervon (subscriber, #722) [Link]
No, actually, a metadata-only sync would *increase* the chance of causing problems over not using anything at all. The necessary operation is to *prevent* writing the metadata until the data has been written.
Ext4 filesystem hits Android, no need to fear data loss (ars technica) Posted Jan 1, 2011 19:22 UTC (Sat) by butlerm (subscriber, #13312) [Link] The necessary operation is to *prevent* writing the metadata until the data has been written.That can lead to severe performance issues, unless you implement some sort of multiversion concurrency (a la BSD style "soft updates") on all your metadata. There are better ways.
Not again Posted Dec 28, 2010 16:06 UTC (Tue) by man_ls (subscriber, #15091) [Link] This particular horse has got its share of postmortem beatings before, but there it goes: what developers usually want is atomicity, not durability. Durability (once written stay written) is a different requirement, but in this particular instance we need atomicity: do the rename in one step, so it's either finished or not done. The same goes for appending to an existing file: either append or do not append, but at no point in the process leave a corrupt file.The funny part about T'so's fsync obsession is that he considers fsync to be a requirement for good filesystem programming. Instead of solving the atomicity problems and shutting up, he insists on the use of fsync: it's unlikely that Android devices will routinely run into the kind of system failure that causes data loss for applications that don't properly use fsync.and proposes "product QA" as a substitute for proper filesystem-level atomicity. Weird.
Not again Posted Dec 28, 2010 17:29 UTC (Tue) by mjg59 (subscriber, #23239) [Link]
Indeed. It's ironic that the outcome of optimising certain performance characteristics of a filesystem would have required userspace to behave in a way that's almost precisely pathological on the previous generation of the same filesystem, but the rational approach to this is "Optimise filesystems for the way that real-world applications work" not "Optimise filesystems for a benchmark". POSIX doesn't provide a mechanism for atomicity without durability, so if application developers want one without the other then they're going to have to rely on implementation details. Filesystems that don't understand that are doomed to irrelevance.
Not again Posted Dec 29, 2010 6:42 UTC (Wed) by Nick (guest, #15060) [Link]
I don't know that developers want atomicity but not durability. I'm not quite sure if developers know what they want, most of the time. At least, there seem to be a lot of misconceptions and misuse of these things. I don't think SQL provides atomicity without durability, does it? (maybe an RDBMS specific extension, but I'm not aware of them).
It's true that posix filesystem API is not trivial to write a crash recoverable protocol on top of, when you are talking about multiple files and directory structure, but it's not rocket science. I think a lot of userspace developers believe that it is terribly difficult and non-performant to implement in their apps, but it would somehow become free of complexity and cost if implemented in the kernel.
Not only would it not be free, but it would add a lot of complexity and divergence (between fses, other OSes) to a layer that is currently quite simple and used by all sorts of apps that do not need such semantics.
I'm not saying it would be a useless feature. But I have not seen anywhere somebody show code and numbers showing that it is compelling, over alternatives. Alternatives include implementing your own atomicity/cleanup protocol on the posix API (which really isn't too hard, and can absolutely include asynchronous writeback until durability is required), or using sqlite or bdb or something to either store all your data or at least store a log of the state of your file/directory structure. And mind you, the alternatives are very well tested and will work on all OSes.
This is funny... Posted Dec 29, 2010 8:47 UTC (Wed) by khim (subscriber, #9252) [Link] The most popular SQL database in the most popular mode only offers atomicity but not durability. The fact that it remains the most popular despite that says volumes about what peeple need (as opposed to want). People ask for durability when they need atomicity - but that's because, as you've said, usually developers don't know what they need (they usually know what they want, but that's different thing).
This is funny... Posted Dec 30, 2010 12:16 UTC (Thu) by Nick (guest, #15060) [Link]
Well I thought MyIASM lack of ACID was widely derided and apparently updates weren't even atomic versus crash recovery, and they now default to using innodb engine which is full ACID.
But the article didn't really mention durability. It mentioned lack of integrity, whereas durability seems like it was implied ("MyISAM tables effectively always operate in autocommit = 1 mode").
You can definitely turn off fsync on RDBMS which I think is used to do bulk populates of a new database. But there is definitely that point of populating the thing to transitioning to live data when you need a durable point too. But this is because crash recovery at this point is trivial (recreate the db) compared to the larger cost of fsync.
Not again Posted Dec 29, 2010 13:04 UTC (Wed) by man_ls (subscriber, #15091) [Link] It's not that developers don't want durability: quite often they do, for example after saving a file to disk. But in those instances it is safe and clean to provide it by using fsync(). I don't care if my application pauses for a few seconds after saving a file to disk, but at that point I want durability. I don't want it to stop every minute when it saves an emergency backup file, but neither do I want it to corrupt said emergency file; at those moments I want atomicity.I was probably not clear enough when speaking about "developers", since they are not here the determinant force here but merely a vehicle for users needs. Users want robust filesystems which do not corrupt their data; application developers just want to do their stuff without being bothered by the underlying filesystem. It is filesystem developers which need to provide for those seemingly disjoint requirements by providing atomicity always (again, merely a vehicle for robust filesystems) and durability sometimes (only when asked for explicitly). If POSIX does not require atomicity in renames and appends, then are filesystem developers free to corrupt user files and claim to be POSIX-compliant? Ask XFS developers, which did corrupt files once in a while for a few years and lost most of their users in the process, what good this compliance does. If ext4 did this then the whole Linux ecosystem would suffer in the process as it is the default filesystem. Implementing filesystems on a database-like layer is what Microsoft Longhorn set out to do, one of the reasons why it was delayed some six years, and also why the final Windows Vista was so utterly bad: a lot of resources were spent in what was, simply put, bad engineering. I would have said "in retrospect" but for many people it was obviously a mistake from the beginning. There have been a few other contenders in the filesystem-on-a-databse category and IIRC all failed.
Not again Posted Dec 30, 2010 12:07 UTC (Thu) by Nick (guest, #15060) [Link]
Then you use the normal posix API to write your emergency backup files. I still fail to see what is missing here.
Write the file (to a new name, obviously, don't overwrite the old one until the new one is durable), and optionally kick off an asynchronous writeback of that file. At some point in future when durability is required (eg. if you guarantee some window or interval on your backed up data), then you would fsync (which should be mostly noop if the asynchronous writeback already completed). And then perform some checkpoint such as a lock file or sqlite transaction or something that indicates the file is now durable.
It's really not a big deal to implement some simple transactional-like recovery protocol on top of the POSIX API, and it has been done in these ways for a long time. It seems like a new wave of programmers has just forgotten about this.
Or, if you want to do slow, synchronous operations without blocking the whole app, that's trivial too -- just use processes or threads to do those slow operations.
Not again Posted Dec 30, 2010 13:43 UTC (Thu) by man_ls (subscriber, #15091) [Link] What is missing is that you don't really want all the files you write to be durable, even if you think you do: many of them are just temporary files which won't perhaps be needed in the future, and for the rest the write operation can be delayed a few seconds with no ill effects. You just want your renames to be atomic: either keep the old file or overwrite it with the new one. And the same with appends. It's not really important which version (old or new) is left on disk, but do never leave corrupted files (empty files or files with unitialized sectors).You can paper over these inefficiencies with new APIs; you can require all developers to use them before you make guarantees about not corrupting data; or you can force them to use threads or processes. But those are not very elegant ways to deal with the problem. I prefer to have a filesystem which silently deals with the problem without corrupting my data, like we did with ext3 in the good old times. But once again, please read the original articles where these issues were truly beaten to death.
Not again Posted Dec 31, 2010 1:00 UTC (Fri) by Nick (guest, #15060) [Link]
If you don't need files to be durable, then don't fsync them!
If there is a crash, then you don't have to bother with atomicity, just delete the files (which is possible because you didn't require durability).
These aren't inefficiencies, they are actually efficiencies, because they expose an API that is simple and efficient to implement for the memory management and filesystems. You can build arbitrarily more complex protocols on top of that.
And yet again here we are Posted Dec 31, 2010 14:08 UTC (Fri) by man_ls (subscriber, #15091) [Link] What I want is not a durable file, but an atomic rename. Not an empty file, but either the old version or the new.Apparently the atomic rename is too confusing, or I am not explaining the use case clearly. Think about atomic appends to files then. When you append some sectors atomically, you want either the old file without the new sectors or the file with the new sectors at the end. What XFS did (and I experienced first hand) was add some uninitialized sectors to the file, and afterwards write the new content to the sectors. This behavior is apparently allowed by POSIX, and yet extremely annoying to users, who moved to other filesystems in droves. An atomic append would guarantee that the new sectors would either be present with the expected contents or not present at all, but never contain random garbage. We don't need an fsync after every sector, nor would it solve the problem: after the write but before the fsync the filesystem would be in an incoherent state anyway, albeit for a short time.
And yet again here we are Posted Jan 5, 2011 12:03 UTC (Wed) by nye (guest, #51576) [Link]
>What XFS did (and I experienced first hand) was add some uninitialized sectors to the file, and afterwards write the new content to the sectors. This behavior is apparently allowed by POSIX, and yet extremely annoying to users, who moved to other filesystems in droves.
To be fair there was more wrong with XFS than that - it could also corrupt *entirely unrelated* files. Personally I stopped using it when a power cut trashed /etc/passwd (and presumably a load of other files that were less obvious) despite nothing having it open at the time.
Not again Posted Dec 31, 2010 16:08 UTC (Fri) by etienne (subscriber, #25256) [Link]
How about, when you want to save the new configuration file, not trying to replace the old config file - just keep it in case the new config file is incomplete, still not created or corrupted (after crash)?
Should work with any filesystem.
Not again Posted Dec 31, 2010 16:21 UTC (Fri) by man_ls (subscriber, #15091) [Link] And, the next time? Would you keep all the old files? You can also keep two of them, but how do you know which one of them was the last one written? What if one of them is half-written? That way leads to madness. Just to avoid a feature which has worked on ext3 (and probably all other popular POSIX filesystems) for decades, and which other OSs are trying to copy.
Not again Posted Dec 31, 2010 19:05 UTC (Fri) by etienne (subscriber, #25256) [Link]
> Just to avoid a feature which has worked on ext3 for decades
Well, for decades you did not have hard drives with internal command queueing.
Once again, reenacting history Posted Dec 31, 2010 20:21 UTC (Fri) by man_ls (subscriber, #15091) [Link] It didn't happen that way. I did explicitly use a journaling filesystem thinking that "journaling" meant that it did not lose data in the event of a crash. When I found out that only metadata was guaranteed to be consistent, I thought "What a sham". Then I (and millions of other people) immediately switched to another filesystem: ext3 with data=ordered, which did offer better guarantees (data journaling) at the cost of performance. Who wants performance when your files are being corrupted?The funny thing is that XFS developers eventually realized their folly and solved the atomicity issues, but now people don't trust them with their data anymore.
Not again Posted Dec 31, 2010 22:14 UTC (Fri) by neilbrown (subscriber, #359) [Link]
1/ ext3 is the only filesystem I know of that forces and fsync before committing a rename.
2/ ext3 is about 10 years old, so it hasn't been around for "decades" unless you mean "0.9 decades". 3/ When rename was first introduced into Unix in the BSD, it was atomic in the sense that even in the event of a crash there would always be a file with the destination name, either the original or the new. This is in contrast to the previous behaviour. which required: - create "file.tmp" - unlink "file" - link "file.tmp" to "file" - unlink "file.tmp"
which can easily leave nothing called "file". This is all that "atomic rename" means, or at least all it meant before ext3 gave rename unfortunate (though useful) semantics.
Though I cannot know the intention of the author of that post you linked to, there is no prima-face reason to believe they mean anything more than the atomicity of names (not of contents) that rename has always had in Unix.
(and half-written files are easy to detect by writing a checksum at the end. If you suffix each file with a timestamp it is easy to know which is the most recent. And file older than a few minutes will be safe-on-disk so you are always free to clean up any file older than the youngest file that is older than a few minutes)
|
|||||||||||||