On 07/07/2015 09:01 PM, Wes Vaske (wvaske) wrote:
Regarding:
“lie about their fsync status.”
This is mostly semantics but it might help google searches on the issue.
A drive doesn’t support fsync(), that’s a filesystem/kernel process. A drive will do
a FLUSH CACHE. Before kernels 2.6.<low numbers> the fsync() call wouldn’t sent
any ATA or SCSI command to flush the disk cache. Whereas—AFAICT—modern kernels and
file system versions*will* do this. When ‘sync’ is called the filesystem will issue
the appropriate command to the disk to flush the write cache.
For ATA, this is “FLUSH CACHE” (E7h). To check support for the command use:
[root@postgres ~]# smartctl --identify /dev/sdu | grep "FLUSH CACHE"
83 13 1 FLUSH CACHE EXT supported
83 12 1 FLUSH CACHE supported
86 13 1 FLUSH CACHE EXT supported
86 12 1 FLUSH CACHE supported
The 1s in the 3rd column represent SUPPORTED for the feature listed in the last
column.
Right, to be precise, the problem isn't the drive lies about fsync(). It
lies about FLUSH CACHE instead. Search & replace fsync() with FLUSH
CACHE, and the same question remains: When the drive breaks its promise
wrt. FLUSH CACHE, does it nevertheless guarantee that the order the data
is eventually flushed to disk is consistent with the order in which the
data and FLUSH CACHE were sent to the drive? That's an important
distinction, because it makes the difference between "the most recent
data the application saved might be lost even though the FLUSH CACHE
command returned" and "your filesystem is corrupt".
- Heikki
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