Re: Disk write cache (Was: Hyper-Threading Vulnerability)

Gene Heskett
2005-05-16T03:06:04Z

On Sunday 15 May 2005 22:24, Mikulas Patocka wrote:
>On Sun, 15 May 2005, Gene Heskett wrote:
>> >There is a large amount of yammering and speculation in this
>> > thread.
>>
>> I agree, and frankly I'm just another of the yammerers as I don't
>> have the clout to be otherwise.
>>
>> >Most disks do seem to obey SYNC CACHE / FLUSH CACHE.
>> >
>> > Jeff
>>
>> I don't think I have any drives here that do obey that, Jeff. I
>> got curious about this, oh, maybe a year back when this discussion
>> first took place on another list, and wrote a test gizmo that
>> copied a large file, then slept for 1 second and issued a sync
>> command. No drive led activity until the usual 5 second delay of
>> the filesystem had expired. To me, that indicated that the sync
>> command was being returned as completed without error and I had my
>> shell prompt back long before the drives leds came on. Admittedly
>> that may not be a 100% valid test, but I really did expect to see
>> the leds come on as the sync command was executed.
>>
>> I also have some setup stuff for heyu that runs at various times
>> of the day, reconfigureing how heyu and xtend run 3 times a day
>> here, which depends on a valid disk file, and I've had to use
>> sleeps for guaranteeing the proper sequencing, where if the sync
>> command actually worked, I could get the job done quite a bit
>> faster.
>>
>> Again, probably not a valid test of the sync command, but thats
>> the evidence I have. I do not believe it works here, with any of
>> the 5 drives currently spinning in these two boxes.
>
>Note, that Linux can't send FLUSH CACHE command at all (until very
> recent 2.6 kernels). So write cache is always dangerous under
> Linux, no matter if disk is broken or not.
>
>Another note: according to posix, sync() is asynchronous --- i.e. it
>initiates write, but doesn't have to wait for complete. In linux,
> sync() waits for writes to complete, but it doesn't have to in
> other OSes.
>
>Mikulas
>
Humm, I'm getting the impression I should rerun that test script if I
can find it. I believe the last time I tried it, I was running a
2.4.x kernel, right now 2.6.12-rc1.

--
Cheers, Gene
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