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Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
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fallbacks for testing and development
Posted Apr 21, 2008 15:24 UTC (Mon) by Jel (subscriber, #22988) [Link]
I seem to recall trying LVM on loopback files, and being shocked to find out that it's not supported. I never got why LVM doesn't allow this. Isn't that the whole unix philosophy -- to treat everything as a file? To allow for unusual scripting and unexpected setups? Wouldn't it be perfect for both benchmarkers and developers to be able to flexibly test these things on top of a single hard drive (or even a ram drive) which has known performance characteristics? I think every linux feature that uses hardware should have software or filesystem fallbacks. That way IM client developers, media player developers could write for all sorts of webcam and TV card and genlocker and surround sound setups without actually having that hardware. I only need to barely mention the security researchers setting up honey pots, the admins doing deployments, etc. for people to see that there's a real need for this sort of flexibility. Just think of how many times you've mounted a filesystem as loopback, simulating a physical drive. This should be possible across the whole platform, with every device. I know that's a tall order, but we should recognise the principle and move towards it, at least.
No LVM on loopback
Posted Apr 22, 2008 1:40 UTC (Tue) by pr1268 (subscriber, #24648) [Link]
I never got why LVM doesn't allow this. Isn't that the whole unix philosophy -- to treat everything as a file? To allow for unusual scripting and unexpected setups?
I'm curious about that, too. I thought that the whole VFS kernel subsystem was to separate all the layers of the file system to enable all sorts of different HW and FS combinations1. But, I do take comfort in knowing that, if it doesn't exist currently, then there's probably a good reason why it doesn't exist.
1 I'm still amazed at how easy it was to set up a Linux kernel software RAID-0 on two IDE disks running off of separate ribbon cables attached to a Promise ATA controller card that came free with my purchase of a WDC 200GB hard drive four years ago. As unusual as I consider this setup to be, Linux gives me absolutely no grief about it (and this virtual drive hauls ass!). ;-)
No LVM on loopback
Posted Apr 23, 2008 12:55 UTC (Wed) by daniel (subscriber, #3181) [Link]
I have set out to rewrite Linux LVM (lvm3 project, a merge of device mapper capabilities with the generic block layer) and I will certainly ensure that the result works properly on loopback files. Anybody have more wishes? Daniel
No LVM on loopback
Posted Apr 23, 2008 19:55 UTC (Wed) by nix (subscriber, #2304) [Link]
Useful snapshots: i.e. >32 of them, snapshots where snapshotting the root filesystem works... ideally a frontend command like chroot but which snapshots everything below some tree, kicks up a shell in it, and zaps all the snapshots when the shell exits. (Why? So I can back my whole system up via snapshots, reliably. I might well have >32 filesystems: I obviously want to back up / without running a deadlock risk: and the chroot-in-snapshot command is just obviously useful.) So far LVM snapshots have enough limitations that I haven't found a single use for them in practice... it would be nice to have snapshots that work. (Of course we might all go to btrfs and get them for free!)
No LVM on loopback
Posted Apr 23, 2008 21:01 UTC (Wed) by zlynx (subscriber, #2285) [Link]
I use LVM on a Fedora Core system at work. I made a little script to snap each of the filesystems, including the root fs, and then mount each snap in a snapshot directory. That script runs just before the nightly backup job which reads off the nightly snapshot. The script also keeps a weekly snapshot. The two snapshots are very convenient for grabbing quick copies of old files. I also took advantage of the ability to assign new LVs (which a snapshot is) to particular PVs so as to distribute the write load across several disks. Because, when a snapshot source is written to, copies of the old blocks have to be written into the snapshots before the old block can be overwritten on the source. That sped things up a lot. In a couple years of doing this I've never had that system lock up during a backup job, so the risk of deadlock must be very low.
No LVM on loopback
Posted Apr 24, 2008 20:32 UTC (Thu) by nix (subscriber, #2304) [Link]
It is, but still... the problem is apparently that the lvm tools don't lock themselves in memory, and there's a window where I/O to the snapshot origin is disabled. If the OS needs to page in the LVM tools at that point, deadlock. (And that's only the known deadlock.)
No LVM on loopback
Posted Apr 24, 2008 5:07 UTC (Thu) by muwlgr (guest, #35359) [Link]
I would wish :
- raid1/raid5 facilities for separate LVs, unlike current situation
when you have first to assemble the whole RAID PV
in underlying layer by mdadm.
- online filesystem resizing, like jfs/jfs2 in AIX LVM.
There was fsadm script in LVM1, but in LVM2 it has gone.
In short, the final wish really looks like ZFS/Btrfs and of course Zumastor :>
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 21, 2008 18:20 UTC (Mon) by gvy (guest, #11981) [Link]
Rather lame article as he'd do just fine with writeback cache even without BBU -- at least not much worse than with software raid. There's no point in buying 3ware (moreso areca) cards to use them as stupid channels... especially when these are <8ch (there might be some sense with 16ch areca since the cpu's throughput is simply twice not enough).
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 21, 2008 19:24 UTC (Mon) by SanderMarechal (guest, #51684) [Link]
> Rather lame article as he'd do just fine with writeback cache even > without BBU -- at least not much worse than with software raid. True. Performance was roughly equal using mdadm or using 3ware hardware RAID with write cache, but you run the risk of loosing some data on a power loss. I thought I wrote this in the article but apparently I didn't, I have edited it now. My server isn't hooked to an UPS. That, combined with the advantage of the "partition size trick" I mentioned made me go for software RAID, regardless of the small performance differences there would be. > There's no point in buying 3ware (moreso areca) cards to use them as stupid channels... My server has one regular PCI slot and it runs at 33 Mhz. That makes for a total max throughput of 133 MiB/sec. Easily filled by my two SATA 3.0 disks. I can hook up four drives to my card. My PCI-X slot is 64-bit/100 Mhz and caps out at 800 MiB/sec. More than enough for four SATA disks. Yeah, it sucks spending good money on an expensive RAID card and only use it at a channel. But it was the only thing I could find that could run in PCI-X on 100 Mhz. I prefer it to not having the 3ware card at all so I still consider it money well spent :-)
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 21, 2008 21:08 UTC (Mon) by tialaramex (subscriber, #21167) [Link]
I had a not disimilar experience with 4 channel 3ware hardware years ago and wasn't able to get any performance out of it at all in a RAID configuration. Perhaps I just didn't read enough manuals (or make enough paid support calls) to find out what I needed to do to enable this "writeback cache" (I don't remember what settings we did try, like I said, years ago, at the time I had detailed notes). I also found the 3ware software to be abysmal and it seemed to want to wind its tentacles deep into my Linux system which I wasn't very comfortable with. Like the article's writer, I opted for software RAID, using the 3ware card as a fast ATA to SCSI controller, and found it to perform very adequately for my needs. Since then I've skipped hardware RAID controllers altogether in my own machines and wouldn't bother even suggesting them until they become necessary due to e.g. sheer quantity of drives connected. But we do have Dell branded SAS RAID on some servers because someone overrode my intended specification, and it seems to behave itself, at least it's kept out of my way so far. Of course for all I know it's just that the monitoring doesn't work, and in fact one of the drives died this weekend and the other will cop out tonight... I have suffered a series of nasty failures on a PC that I eventually tracked down to a failing IDE controller. All the contents were under Linux Software RAID, and it soldiered on through a series of scary-looking IDE errors, I still have both drives which seem to work fine with the dodgy controller out of the picture (it eventually refused to recognise hard disks on boot, at which point I realised what was going on and replaced the whole board) and the array was rebuilt each time with no significant data loss, just an ext3 journal replay needed and a few hours of reduced disk throughput. That array is now retired because I switched to larger SATA disks.
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 21, 2008 20:33 UTC (Mon) by maney (subscriber, #12630) [Link]
Apparently the only problem with running RAID on the 3ware was that they warned him of potential data loss on power loss without the battery. This is not much, if at all, mitigated by running the card as a dumb channel, or with RAID enabled and disk caches off, since there's still plenty of deferring of writes going on (hint: that's why Linux runs better when there's RAM available for caching disk blocks). Running any sort of RAID, hardware or soft, in hopes of getting better reliability without putting the whole setup on an UPS with signalling to allow an intentional shutdown rather than an unplanned shutoff is, IMO, pretty pointless, power glitches being so much more common than hard drive failures. Just sayin'
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 21, 2008 22:17 UTC (Mon) by SanderMarechal (guest, #51684) [Link]
That's why I want a UPS in my next house, when I have the space to put it all in (and a 19" rack - but that's partly for geek points :-)
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 22, 2008 6:55 UTC (Tue) by topher (guest, #2223) [Link]
I've gotta disagree with you slightly here, with regards to RAID without UPS being pointless. While it's true that power glitches are significantly more common than hardware failures, they're also significantly easier to recover from. Minor data loss due to a power outage is a lot better than complete data loss due to the failure of a non-RAID disk.
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 22, 2008 14:59 UTC (Tue) by Thalience (subscriber, #4217) [Link]
The kernel's use of deferred writes (held in system ram) is greatly mitigated by the use of a journaling filesystem. Such filesystems have a large amount of code dedicated to ordering the writes so that recovery from a power failure is easy. A volatile write cache on a raid card or individual disk breaks the filesystem's assumptions about when its write commands are actually complete. The situation with individual drives is getting better, as more of them are supporting various types of cache flush commands, or tagged writes. The cache on a typical hardware raid card is too large to be flushing on every journal commit, but I don't see why tagged write commands wouldn't be very useful. However, I am not aware of a card that implements them (or any other way for the kernel to manage its cache).
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 25, 2008 10:40 UTC (Fri) by jschrod (subscriber, #1646) [Link]
While I agree in principle with your comment that one should use UPS together with RAID, there is... > power glitches being so much more common than hard drive failures. Ug. Where are you living? Here in Germany, near Frankfurt, the next hard drive failure is more probable than the next power glitch -- I had several drive failures the last two years, but no power glitch at all.
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted May 3, 2008 19:18 UTC (Sat) by anton (guest, #25547) [Link]
The write caching by the file system is no problem for the file system and data consistency if the file system writes the data out in the right order (and file systems do that (some better and some worse), through such techniques as journaling and/or ordered writes).The write caching by the drives or a hardware RAID controller can be a problem, if it results in the writes being reordered; and at least drives perform such reordering. There are a several remedies: 1) Turn off write caching in the drive (costs a lot of performance if you don't have tagged commands) 2) Use flush commands to the drive at appropriate moments. I don't know if the file systems do these things and do them properly. Last I looked, they did not turn off write caching, but left that to the sysadmin.
How does that mix with RAID1? In case of a power failure, the drives may not contain the same data (one may have written one or more blocks that the other has not written); what the md driver does in this case is to run the RAID from on drive, and sync the other disk from it. I guess that opens a window of vulnerability while the syncing is going on, but it's probably still the safest thing to do. Having a reliable UPS with automatic shutdown would eliminate this window, but the window is typically small compared to the time when the RAID1 protects against a disk failure, so the reliable UPS may not be worth the cost. If the file system was better integrated with the RAID (ZFS?), this situation could be handled in a better way.
Finally, in our experience UPSes are not more reliable than our power grid. We had an UPS die (and take the machine it was supposed to serve down) while the grid was ok and all other machines (including those without UPS) continued working; we eventuelly decided to go without UPSes. The alternative would have been to have redundant power supplies with redundant UPSes, which was not worth the effort for us.
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 21, 2008 23:19 UTC (Mon) by kev009 (guest, #43906) [Link]
He's worried about data loss from power failure but then chose to use XFS... FWIW I get decent performance out of a 3ware 9000S with write cache enabled and no BBU. The system is on a UPS, backups are present, and it is just a light duty home server.
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 22, 2008 12:18 UTC (Tue) by dgc (subscriber, #6611) [Link]
> He's worried about data loss from power failure but then chose > to use XFS... FWIW, I think your information is out of date. We put fixes into 2.6.17 that worked around the problem that caused "data loss on power fail" pretty effectively, and we fixed it properly in 2.6.22. I haven't seen a bug report for "NULL files" on power loss or crash since 2.6.17.... If it's still happening to you on a recent kernel then please report it. Otherwise, you need to find some other reason not to use XFS. ;)
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 22, 2008 16:15 UTC (Tue) by bronson (subscriber, #4806) [Link]
Wow, this is big news! I'm surprised I never heard this. I suggest you put it at the very top of your readme or something: XFS NO LONGER CAUSES DATA LOSS ON POWER FAIL. REPEAT. XFS NO LONGER CAUSES DATA LOSS ON POWER FAIL. Because pretty much everyone I know is still laboring under this misconception!
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted Apr 23, 2008 16:46 UTC (Wed) by alankila (subscriber, #47141) [Link]
I'm thoroughly ignorant on this issue. I thought that basically all journalling filesystems guarantee only that there is consistency of filesystem after service interruptions. Was the nature of XFS's empty files problem somehow more severe than you'd expect to get in other filesystems? I mean, data only in system RAM is lost no matter what. But once it's partially or fully in journal, it should have been preserved. Did XFS not recover file data for situations where it could have done so?
Benchmarking Linux filesystems on software RAID 1 (Lone Wolves)
Posted May 3, 2008 18:42 UTC (Sat) by anton (guest, #25547) [Link]
The way I have heard it, XFS used to zero files that were partially written. I have been bitten by such behaviour, although with a different file system. In my case I lost an hour of work.