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Improving ext4: bigalloc, inline data, and metadata checksumsImproving ext4: bigalloc, inline data, and metadata checksumsPosted Dec 12, 2011 18:14 UTC (Mon) by dlang (✭ supporter ✭, #313)In reply to: Improving ext4: bigalloc, inline data, and metadata checksums by andresfreund Parent article: Improving ext4: bigalloc, inline data, and metadata checksums
no, jlokier is right, barriers are still needed to enforce ordering
there is no modern filesystem that waits for the data to be written before proceeding. Every single filesystem out there will allow it's writes to be cached and actually written out later (in some cases, this can be _much_ later)
when the OS finally gets around to writing the data out, it has no idea what the application (or filesystem) cares about, unless there are barriers issued to tell the OS that 'these writes must happen before these other writes'
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Improving ext4: bigalloc, inline data, and metadata checksums Posted Dec 12, 2011 18:15 UTC (Mon) by andresfreund (subscriber, #69562) [Link]
The do wait for journaled data uppon journal commit. Which is the place where barriers are issued anyway.
Improving ext4: bigalloc, inline data, and metadata checksums Posted Dec 12, 2011 18:39 UTC (Mon) by dlang (✭ supporter ✭, #313) [Link]
issueing barriers is _how_ the filesystem 'waits'
it actually doesn't stop processing requests and wait for the confirmation from the disk, it issues a barrier to tell the rest of the storage stack not to reorder around that point and goes on to process the next requrest and get it in flight.
Improving ext4: bigalloc, inline data, and metadata checksums Posted Dec 12, 2011 18:53 UTC (Mon) by andresfreund (subscriber, #69562) [Link]
Err. Read the code. xfs uses io completion callbacks and only relies on the contents of the journal after the completion returned. (xlog_sync()->xlog_bdstrat()->xfs_buf_iorequest()->_xfs_buf_ioend()).
jbd does something similar but I don't want to look it up unless youre really interested.
It worked a littlebit more like you describe before 2.6.37 but back then it waited if barriers were disabled.
Improving ext4: bigalloc, inline data, and metadata checksums Posted Dec 13, 2011 13:35 UTC (Tue) by nix (subscriber, #2304) [Link]
Well, this is clear as mud :) guess I'd better do some code reading and figure out wtf the properties of the system actually are...
Improving ext4: bigalloc, inline data, and metadata checksums Posted Dec 13, 2011 13:38 UTC (Tue) by andresfreund (subscriber, #69562) [Link]
If you want I can give you the approx calltrace for jbd2 as well, I know it took me some time when I looked it up...
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