Re: [squid-users] Performance question

On Thu, 30 Jun 2005, Matus UHLAR - fantomas wrote:

> BTW I think I've found another reason why RAID should not behave better with
> squid: even SCSI disks behave better on sequential reads. So, fetching a
> file sequentially from a drive (case of multiple filesystems on multiple
> drives) should be faster than fetching some parts (size of a stripe) from
> one disk and some parts from second disk

Luckily for RAID0/5 the stripe size often practically eleminates this
drawback as the percentage of small reads crossing a stripe boundary is
fairly small.

For larger files (but not so for Squid) with little or no other I/O
activity at the same time the above really gives a benefit as you will
then have sequencial reads from more than one drive in parallell, and
similarily for very large writes.

RAID0 does not give any performance benefit for Squid as Squid already
distributes the load among the cache_dirs. You only gain easier
configuration thanks to a single large drive at the expence of loosing the
whole content should one drive fail.

RAID5 of three drives with a good battery backed up HW raid controller is
quite well balanced between performance and redundancy. But don'w confuse
this with "fake" raid controllers without battery backup as these does not
provide the same performance benefits and you get all the negative
performance impacts of RAID5 only gaining redundancy.

Direct mounting of the drives is generally the absolutely fastest, via the
raid controller battery backup is you have one. But lacks in redundancy
should a drive fail. RAID1 of OS and configuration recommended for
redundancy, but somewhat costly for cache drives..

Regards
Henrik
Received on Thu Jun 30 2005 - 04:07:10 MDT