Re: (reiserfs) Re: Raid resync changes buffer cache semantics ---not good for journaling!

[Stephen C. Tweedie]

Hi,

On Thu, 4 Nov 1999 19:03:18 +0100 (CET), Rik van Riel
<[EMAIL PROTECTED]> said:

> The obvious solution would be to allow multiple versions of the
> same disk block to be in memory. 

That is already possible.  You can make as many buffer_heads as you want
for a given disk block, as long as only one is in the buffer cache itself.

> We need that anyway for journalling, so why not extend it with a flag
> (or use another flag) to show that _this_ particular disk block is the
> one on disk or the one supposed to be on disk (go on, flush this).

That's just not the point --- even for the copy which _is_ supposed to
be authoritative on disk, you can't be sure that it is in fact uptodate
because there is a race between the filesystem modifying the buffer
contents and the dirty bit being marked in the buffer_head.

--Stephen