I received responses from
Brenden Phillips <B.C.Phillips_at_massey.ac.nz>
Dirk Grunwald <grunwald_at_foobar.cs.colorado.edu>
alan_at_nabeth.cxo.dec.com (Alan Rollow)
chris_at_lagoon.meo.dec.com (Chris Jankowski)
and would like to thank them for their input.
SUMMARY
Adding more memory to the existing 4Meg on-board now will probably
not gain any performance. The Mylx DAC960 does not have a viable
bater backup for its cache so write-back caching is unusable.
======================================================================
From: Brenden Phillips <B.C.Phillips_at_massey.ac.nz>
To: "Lance A. Brown" <brown9_at_niehs.nih.gov>
Subject: Re: Add memory to Mylex DAC960 PCI SCSI?
> Will adding memory to a Mylex DAC960 PCI SCSI adapter improve
> performance of a RAID-5 disk set used as a USENET news article spool?
Probably - it can't hurt! May be better to look at the config that
you have on the disk. UFS or ADVFS? If its ADVFS then it will run
poorly if you didn't create the file domain and fset with non-default
parameters. There will be something in the archives on this, cause I
seen it go past.
INN and like news servers store all their articles in those lovely
inode intensive directory structures. We shifted our news to dnews
(
http://netwinsite.com/dnews.htm), which is a sucking feed, but it
stores its articles in 10MB DB files, which seems to make the disk
usage much better.
======================================================================
To: "Lance A. Brown" <brown9_at_niehs.nih.gov>
Subject: Re: Add memory to Mylex DAC960 PCI SCSI?
From: Dirk Grunwald <grunwald_at_foobar.cs.colorado.edu>
Probably the bigger difference would be using battary backup and
enabling write-back caching.
======================================================================
From: alan_at_nabeth.cxo.dec.com (Alan Rollow - Dr. File System's Home for Wayward Inodes.)
To: brown9_at_niehs.nih.gov
Subject: Re: Add memory to Mylex DAC960 PCI SCSI?
I've seen performance reports that suggest it takes surprising
little cache to improve write performance unless the write load
is all sequential. With News the write load is likely to be
fairly small transfers (often whole files in a single transfer)
and the News protocol and network delays will ensure that there
is some idle time for the cache manager on the controller to flush
the cache. Large articles may still cause the cache problems, but
such I/O loads are also likely to allow the controller the chance
to optimize the write patterns.
While I don't have any proof (that's what benchmarks are for) I
would tend to believe that more cache won't help much. Your
biggest problem is that the tendency for writes to be small,
is exactly the wrong I/O load for RAID-5. But, you apparently
already knew that...
======================================================================
From: chris_at_lagoon.meo.dec.com (Chris Jankowski)
To: brown9_at_niehs.nih.gov
Subject: Add memory to Mylex DAC960 PCI SCSI?
No.
It does not make much sense to add more memory to your Mylex.
The main reason is that this is only write through cache
as it does not have a viable battery backup.
Viable means here one that can report when the battery is broken.
If you have performance bottlenecks due to reads just double
the size of the memory on your system.
Your buffer cache in your computer memory is bigger and it does a better
job than the pitiful 4Mb on Mylex.
RAID may still be a viable proposition for news spool if you use
a high quality (and thus higher priced) controller.
HSZ50 (or HSZ70) with 128MG write back cache with viable battery backup
may do wonders for for the synchronous writes incurred during feeds
and cleanups.
Both can be used in conjunction:
Large system memory to have lots of news cached in memory
*and* large write through cache to speed up synchronous
writes during feeds and cleanups.
As an alternative to large write through cache on RAID controller
you may use PrestoServe and selectively only cache the metadata
of the news filesystem.
Unfortunately, all of those cost money.
You may also reconfigure your raidsets as JBOD (and loose availability)
or add more disks and configure them as mirrors.
Received on Tue Nov 25 1997 - 20:06:12 NZDT