Toby Thain wrote:
On 10-Feb-09, at 10:36 PM, Frank Cusack wrote:
On February 10, 2009 4:41:35 PM -0800 Jeff Bonwick
<jeff.bonw...@sun.com> wrote:
Not if the disk drive just *ignores* barrier and flush-cache commands
and returns success. Some consumer drives really do exactly that.
ouch.
If it were possible to detect such disks, I'd add code to ZFS that
would simply refuse to use them. Unfortunately, there is no reliable
way to test the functioning of synchonize-cache programmatically.
How about a database of known bad drives? Like the format.dat of old.
The intransigence of disk makers is incredible. Name and shame might
work, though.
I do like the idea of a 'known bad' DB, just a quick reference for people to
check on and drop an email to $vendor indicating someone's added $drive to the
list based on $test ? It's a lot of work to keep updated though. :-/
JB> because it is *impossible* to know when the data is on stable storage.
Pardon the ignorance to in-depth drive internals for a moment, would it be
possible to time a write of X to the drive, time a write of X to the drive again
w/ a sync, power it off immediately after the sync returns (physically ?
programmatically ?) then back on to re-read data that was just written ? If
it's there, then the sync didn't lie, otherwise the drive failed the test. Many
BIOS support powering off the machine on shutdown, could the same command be
issued to hose the drive in this scenario skipping a 'proper' shutdown procedure
? Or would the PSU continue supplying it with power long enough for it to finish
writing ? I suppose it would have to be a sufficiently large write...
Alternatively, timing the tested writes across various sectors of the disk would
give you a good baseline of how long writes take. Would forcing a sync
immediately after the writes to the same locations give you an indication if the
sync was doing as it is supposed to ? If there's a noticeable (*vague) increase
in delay then we assume the sync worked ?
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