Write cache, is write cache, is write cache?

[Olivier Smedts]

2011/1/24 Šimun Mikecin <numisemis at gmail.com>:
> 2011/1/24 Jeremy Chadwick <freebsd at jdc.parodius.com>
>
>> The term "flush" means many different things.  fsync(2), for example,
>> behaves differently on UFS than it does on ZFS.  People think that
>> "flush" means "guarantee the data written was written to disk", but
>> ensuring an actual ATA/SCSI command completes **and** has had its data
>> written to the platters is an entirely different beast (IMO) than "flush
>> kernel buffers to disk and hope for the best".
>>
>
> AFAIK, BIO_FLUSH should complete when data was written to disk (not before),
> or in the case of battery backed cache: when data was written to battery
> backed cache.
> AFAIK, ZFS doesn't only "flush kernel buffers" (like UFS does), but also
> executes BIO_FLUSH every X seconds (X=5 if my memory serves me well). So,
> you shouldn't loose data that was written before BIO_FLUSH executed.

I thought ZFS also took care of executing BIO_FLUSH after each write
to ZIL, because every bit in ZFS expect writes to be ordered, and some
specific writes to be synced to platters, else your FS is gone if ZIL
is corrupted in some way. The way ZFS works is the reason there's
fsck-like repair tool for it. Of course, this does not work if you
don't use a propre controller or disk, but it should behave that way
for most, no ?

>   With single disks, all I've seen are read/write errors which can't be
>
>> repaired.  "zpool status" will actually show what files got affected as
>> a result of the issue, though sometimes "zpool scrub" needs to be run
>> before this can be detected.
>
>
> Adding redudancy does make your data safer, but if everything is working as
> explained, sudden power-loss shouldn't be the cause of those errors even
> with single disks. Correct me if you think that I'm mistaken.
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-- 
Olivier Smedts                                                 _
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