Every time something is written to disk, parity information must be updated. Using iostat will show just how often writes occur. The same general problem holds true for a mirrored (RAID 1) setup.
Striping (RAID 0) will not likely harm Squid's performance, unless one of the drives dies (trashing the whole cache), but it won't likely help much more than spreading your cache dirs across multiple disks and letting Squid worry about the details. Here is a study (from 1996) on whether RAID is worthwhile: http://www.google.com/search?q=cache:f-rlyGxkQxkJ:archive.nlm.nih.gov/pubs/b angbuck/bangbuck.php&hl=en&lr=&strip=1 Obviously prices have dropped, so cost doesn't factor as much, but the timeless bits are: "For applications where fault tolerance is required, and retrieval speeds can be slower than those available from hardware alone, RAID 5 is the best choice. For RAID 5, wider stripes appear to improve both sequential access speed and concurrent access speed for files in this range." and "For applications where fault tolerance is not important and funds are limited, balancing the load across several volumes without the benefit of RAID management can yield fast retrieval speeds at significant cost savings. We find no reasons for choosing RAID 0 for applications involving small files. The slight performance advantage for sequential file retrieval is offset by the cost of the RAID management capabilities and the reliability risk incurred by distributing each file across several disks." If you have the time and the resources, I'd love to see a study where Squid's cache dir is placed on various RAID arrays (both hardware and software) and performance measured. Chris -----Original Message----- From: William Stucke [mailto:[EMAIL PROTECTED] Sent: Monday, November 29, 2004 1:23 PM To: [EMAIL PROTECTED] Subject: RE: [squid-users] RE: Disk Configuration Adam Aube said: - > The only caution about RAID I have seen is that RAID-5 will kill disk performance with Squid (due to its usage profile) How? Why? Please explain. Just because the FAQ says so doesn't make it true, or explain why. Kind regards, William Stucke ZAnet Internet Services (Pty) Ltd +27 11 465 0700 [EMAIL PROTECTED] -----Original Message----- From: news [mailto:[EMAIL PROTECTED] Behalf Of Adam Aube Sent: 29 November 2004 23:28 To: [EMAIL PROTECTED] Subject: [squid-users] RE: Disk Configuration Chris Robertson wrote: > Everything I've read says that you should not use any RAID for your cache > directories. Make a bunch of cache dirs (each on its own disk), and let > Squid sort it out. The only caution about RAID I have seen is that RAID-5 will kill disk performance with Squid (due to its usage profile). I don't know if RAID-1 and RAID-10 are similarly affected. For best disk performance you should use several smaller disks and have Squid setup a cache dir on each one (no RAID). Adam
