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Linux Storage and Filesystem Workshop, day 2Linux Storage and Filesystem Workshop, day 2Posted Apr 17, 2009 21:10 UTC (Fri) by willy (subscriber, #9762)In reply to: Linux Storage and Filesystem Workshop, day 2 by giraffedata Parent article: Linux Storage and Filesystem Workshop, day 2
That's no more interesting than the simpler problem that when you go to extend a sequential file, the write request fails even though the filesystem has space available.This was in the context of an application having used the posix_fallocate() call. If that call succeeds, the application is guaranteed to be able to use the storage that has been so allocated. Thin Provisioning breaks this, and it's far from clear how to fix it.
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Linux Storage and Filesystem Workshop, day 2 Posted Apr 18, 2009 22:10 UTC (Sat) by giraffedata (subscriber, #1954) [Link] That's no more interesting than the simpler problem that when you go to extend a sequential file, the write request fails even though the filesystem has space available.This was in the context of an application having used the posix_fallocate() call. If that call succeeds, the application is guaranteed to be able to use the storage that has been so allocated. Thin Provisioning breaks this, and it's far from clear how to fix it. No, posix_fallocate() doesn't guarantee you can use the storage. Your write to it could fail, for example, due to a media defect. What posix_fallocate() guarantees is that a write won't fail because there is no space left in the filesystem. That's the same guarantee you get from mkfs that if you create a 1G filesystem, you can put (about) 1G of files in it. So it's equally interesting that with thin provisioning, your fallocated space within a file may be unusable as that your filesystem space may be unusable for extending the file. With thin provisioning, the filesystem storage medium is a volume in the storage server, which does in fact have space for the write in question. What doesn't have space is the server's pool of backing storage, and the effect of that is a write to certain sectors of the volume fails.
Linux Storage and Filesystem Workshop, day 2 Posted Apr 20, 2009 14:35 UTC (Mon) by willy (subscriber, #9762) [Link] The text from the standard says:DESCRIPTIONI stand by my assertion that it is currently impossible to provide this with Thin Provisioning without actually writing to every single block.
Linux Storage and Filesystem Workshop, day 2 Posted Apr 20, 2009 16:05 UTC (Mon) by giraffedata (subscriber, #1954) [Link] That's entirely consistent with what I wrote. There is space in the filesystem. (If there weren't, the write would fail with ENOSPC and statfs() would show no space).The only problem is that writes to certain sectors of the storage medium fail for reasons out of the scope of POSIX. There's layering going on here.
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