On 19/02/2010 21:57, Ragnar Sundblad wrote:
On 18 feb 2010, at 13.55, Phil Harman wrote:
Whilst the latest bug fixes put the world to rights again with respect to
correctness, it may be that some of our performance workaround are still unsafe
(i.e. if my iSCSI client assumes all writes are synchronised to nonvolatile
storage, I'd better be pretty sure of the failure modes before I work around
that).
But are there any clients that assume that an iSCSI volume is synchronous?
Isn't an iSCSI target supposed to behave like any other SCSI disk
(pSCSI, SAS, FC, USB MSC, SSA, ATAPI, FW SBP...)?
With that I mean: A disk which understands SCSI commands with an
optional write cache that could be turned off, with cache sync
command, and all those things.
Put in another way, isn't is the OS/file systems responsibility to
use the SCSI disk responsibly regardless of the underlying
protocol?
/ragge
Yes, that would be nice wouldn't it? But the world is seldom that
simple, is it? For example, Sun's first implementation of zvol was
unsafe by default, with no cache flush option either.
A few years back we used to note that one of the reasons Solaris was
slower than Linux at fileystems microbenchmarks was because Linux ran
with the write caches on (whereas we would never be that foolhardy).
And then this seems to claim that NTFS may not be that smart either ...
http://blogs.sun.com/roch/entry/iscsi_unleashed
(see the WCE Settings paragraph)
I'm only going on what I've read.
Cheers,
Phil
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