Re: RAID1 vs RAID5 [ was Re: 1 processor vs. 2]
On Thu, 4 Mar 2004, Chuck Swiger wrote: Also, RAID-5 performance degrades horribly if a drive is down, whereas RAID-1 does fine... Using the algorithm you indicate below, RAID-5 performance would not degrade on the loss of a drive, it's start out that badly. A five-disk RAID-5 array has to read 4 sectors and write five sectors if you change one byte. Wrong; see previous response. -- jan grant, ILRT, University of Bristol. http://www.ilrt.bris.ac.uk/ Tel +44(0)117 9287088 Fax +44 (0)117 9287112 http://ioctl.org/jan/ Q: What's yellow and equivalent to the axiom of choice? A: Zorn's lemon. ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: RAID1 vs RAID5 [ was Re: 1 processor vs. 2]
Danny Pansters wrote: So statistically and theoreticaly RAID1 compares to no RAID at all as 2x read speed, 1x write speed (it needs to be written twice but through two heads on two drives seperately and assume they react and move at the same speed). That's about right, but you should be aware of the fact that you are only considering a single transaction. When you think about a stream of I/O requests, either round-robin or geometric division of read access can perform better depending on whether you are moving lots or data (few big requests) or doing many small reads and being bound by seek times. Because the heads may be in different places do to distribution of reads, the heads don't end up writing data out at the exact same time; allowing write to be asyncronous rather than requiring both drives to complete a write operation can speed things up, particularly if geometric distribution of reads is being used. Take a RAID5 with 5 drives that would in terms of data resiliance compare with a RAID1 of 3 drives at best (right?). No. A three-disk RAID-1 arry can retain data even if two drives fail. The RAID-5 array will lose data if two drives fail. In more complicated cases (RAID-10 and RAID-50), RAID-1 has significantly greater reliability than RAID-5. Also, RAID-5 performance degrades horribly if a drive is down, whereas RAID-1 does fine... Change the above numbers for a RAID1 to 3 drives and you have a 3x read and a 1x write speed. With the hypothetical RAID5 as above we have 3x read and 1x write speed for data plus 2x read and 1x write for parity info which will usually be smaller in size. Let's assume they're of comparative sizes, to make things simple, then we have 5/2x reads and 1x writes to compare. For your analysis to be valid, you need to consider the size of I/O requests and the stripe or interleave size of the RAID arrays. For example: A five-disk RAID-5 array has to read 4 sectors and write five sectors if you change one byte. 9 I/O ops compared with either 2 or 3, depending on whether one has a two-disk or three-disk RAID-1 mirror. In other words, RAID-1 small writes are often a factor of four times faster than RAID-5 writes for data much smaller than stripe size. Likewise, your analysis of RAID-5 read performance neglects to consider the fact that data is only available on one drive, whereas will be available on two (or more) drives for RAID-1. For small reads, seek performance is critical and RAID-1 performance is close to n times that of a bare drive, where n is number of drives in array, and can sometimes even be slightly higher than n. RAID-5 performance can at best be (n - 1) and is usually closer half that because the I/O distribution potential of mirroring means you can't choose to utilize a drive that's free. Perhaps a better way of putting it is that RAID-1 can decrease response latency by handling reads in parallel, whereas a RAID-5 array cannot because a particular set of data is only available from one place. -- -Chuck ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
RAID1 vs RAID5 [ was Re: 1 processor vs. 2]
On Wednesday 03 March 2004 23:20, Reko Turja wrote: RAID-1 will be about 50% faster than RAID-5 doing reads regardless of size, and will also be *much* faster doing small writes-- by a factor of 4, perhaps. The abovementioned figures seem more like comparing RAID-0 (striping) to RAID-5 (striping with ECC) than RAID-5 to RAID-1 (mirroring). In my experience mirroring is always the slowest RAID in terms of retrieving data, writes might be quite comparable with RAID-1 and RAID-5 though. That makes sense. With more disks you have more disk heads to read with. So statistically and theoreticaly RAID1 compares to no RAID at all as 2x read speed, 1x write speed (it needs to be written twice but through two heads on two drives seperately and assume they react and move at the same speed). Take a RAID5 with 5 drives that would in terms of data resiliance compare with a RAID1 of 3 drives at best (right?). Change the above numbers for a RAID1 to 3 drives and you have a 3x read and a 1x write speed. With the hypothetical RAID5 as above we have 3x read and 1x write speed for data plus 2x read and 1x write for parity info which will usually be smaller in size. Let's assume they're of comparative sizes, to make things simple, then we have 5/2x reads and 1x writes to compare. Therefore reads are poorer and there's more overall CPU/RAM overhead (with hardware RAID this depends on how you look at it, all RAID is essentially software RAID be it on your PC on on a chip inside it). This simplified approach would indicate that 6 or more drives might be a nice thing for RAID5. I thought this over more often when thinking of how to deploy vinum and recently about whether to buy a RAID1 (cheap) or RAID5 ($$) ATA card and always thought this was the correct way to consider performance (resilliance is another thing). Do people agree on this? I often wondered because most of the meta-information around doesn't go into specifics like this. I think the above scenario applies only when there's a reasonable amount of I/O going on concurrently rather than if nothing ever happens but that lone write or read. That would change the assumption that more heads == more reads equal writes (not rights ;-). Sorry to divert a bit. What can I say, I like having a (somewhat informed) discussion... And this is still relevant to OP. Greets, Dan ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
