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Re: Just some odd questions out of the blue...

To: "l.a walsh" <xfs@xxxxxxxxx>
Subject: Re: Just some odd questions out of the blue...
From: Eric Sandeen <sandeen@xxxxxxx>
Date: Thu, 6 Mar 2003 19:59:50 -0600 (CST)
Cc: linux-xfs@xxxxxxxxxxx
In-reply-to: <00a501c2e44a$cfaba280$1403a8c0@xxxxxxxxxxxx>
Sender: linux-xfs-bounce@xxxxxxxxxxx
On Thu, 6 Mar 2003, l.a walsh wrote:

> So lets say you had 2 disks...would it make sense to put the log
> of disk1 on disk2 and the log of disk2 on disk1?  This would be

Yes, external logs are available for this reason.

Your cross-log scenario would probably only help if you were really only
writing to 1 fs at a time.

> Second question ... suppose one disk was faster than the other --
> or one was a sda and the other hda.  How much metadata is written
> compared to file data, i.e. is there some average ratio or range?

You can look at the stats with the xfs_stats.pl script in cvs.
It really depends on the nature of your workload.

> On the assumption that metadata is smaller, seem like one could use
> a slower log disk for a primary work disk, and the slower log disk
> is mostly archival things that aren't written alot, but more read
> alot -- like mp3's, or CD images....things where the slower read isn't
> going to be a big problem.

True, for reads the log speed isn't critical, but for writes again
it will depend on your workload.

> When writing to disks with a cache, does XFS force any flushes (like
> on log data?)  Seem like even if you had a slower disk but an 8Mb
> cache you could keep up with a fairly good write speed on the faster
> disk.

Which is great until you crash, if the cached data is lost...
I don't think xfs explicitly does any ide cache flushing.

> But here's another Q...if you don't flush the on disk cache after
> a log write, then it is 'granted' that the potential for metadata
> loss is at least the size of the on-disk cache.  That could beg
> the question -- would it be of any benefit to write a pseudo-block
> device that lives on top of a disk that just does read-write
> caching -- maybe it lives with a 64Mb Buffer and attempts to use
> geometry knowledge of the disk to optimize head motion, whatever.

Ick.  :)  I think the drive mfgr is the only one who has the knowledge.


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