Write cache, is write cache, is write cache?

[Olivier Smedts]

2011/1/24 Olivier Smedts <olivier at gid0.org>:
> 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

*no*

> 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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-- 
Olivier Smedts                                                 _
                                        ASCII ribbon campaign ( )
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  "Il y a seulement 10 sortes de gens dans le monde :
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  et ceux qui ne le comprennent pas."