Neil Brown <neilb@xxxxxxx> writes:
> On Wed, 24 Feb 2010 09:46:23 -0500
> Bill Davidsen <davidsen@xxxxxxx> wrote:
>
>> > There is no question of data corruption.
>> > When memory changes between being written to one device and to another, this
>> > does not cause corruption, only inconsistency. Either the block will be
>> > written again consistently soon, or it will never be read.
>> >
>>
>> Just what is it that rewrites the data block? The user program doesn't
>> know it's needed, the filesystem, if any, doesn't know it's needed, and
>> as far as I can tell md doesn't do checksum before issuing the write and
>> after the last write is done. Doesn't make a copy and write from that.
>> So what sees that the data has changed and rewrites it?
>>
>
> The filesystem re-writes the block, though probably it is more accurate to
> say 'the page cache' rewrites the block (the page cache is essentially just a
> library of code that the filesystem uses).
>
> When a page is changed, its 'Dirty' flag is set.
> Before a page is written out, the Dirty flag is cleared.
> So if a page is written differently to two devices, then it must have been
> changed after the Dirty flag was clear, so the Dirty flag will be set, so the
> page cache will try to write it out again (after about 30 seconds or at
> unmount time).
So maybe MD could check the dirty flag after write and then output a
warning so we can track down the issue. MD could also rewrite the page
prior to setting the disks in-sync until the dirty bit is clear after a
write.
MfG
Goswin
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