Re: XFS and journalling filesystems

Dan Koren (dkoren@cthulhu.engr.sgi.com)
Tue, 01 Jun 1999 02:06:09 -0700

Jeff Merkey wrote:
>
> Jim,
>
> Some journal based FS's will serialize reads when A) there is cached
> data (in a buffer cache for example), and there is a requirement for
> writes to be committed real time with reads always having the most
> current data, and B) when transactioning is implemented above the
> FS and you have writes and reads to the same block (do you return
> the stale data, or do you return the write that was posted to the
> journal and cached, but not yet committed?) In theory, you could
> avoid serializing on reads, but you may get stale data.

This is not a problem in page cache systems like Irix, Solaris or
SVR4 (UnixWare). File data is kept in the page cache, not in the
buffer cache, and the same page(s) are mapped or accessed by all
the processes that access a file. If a process writes to a file
the new data becomes visible to all readers of the file as soon
as the write system call has completed. Also note that certain
OS's do read/write serialization in the VFS layer anyway, quite
independently of what file systms do underneath (I believe SVR4
and Solaris behave this way).

thx,

Dan Koren Dan.Koren@sgi.com
Engineering Manager, File Systems phone: (USA) 650-933-3678
Silicon Graphics, Inc. pager: (USA) 888-769-0874
1600 Amphiteatre Pkwy. M/S 08U-500 or dkoren_p@pager.sgi.com
Mountain View, CA 94043-1351 fax: (USA) 650-933-3542

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