Hi Benjamin, Thanks for your insight into the reasons why this may be the case, and I agree with your assumptions, I really only added the RAID0 at the end drives with some left over space just to see what RAID0 was like.. One thing I know is that Mandrake makes setting up RAID a joy..
Cheers Mark On Sun, 2002-09-29 at 06:03, Benjamin Pflugmann wrote: > Hi. > > On Sat 2002-09-28 at 20:20:37 +1000, Mark Williamson wrote: > > > > Just trying to workout why I have a Raid 0 that is slower that a > > Raid 1 device.. > > Why do you think that this shouldn't be the case? AFAIK, usually > RAID0 is at most comparable to RAID1 on reading speed, but may be > slower dependend on use. RAID0 is usually faster quite a bit on > writing. > > > Regardless, there are several possible reasons. The two that come to > my mind: > > - speed of different partitions can be different (remember, speed on > inner side and outer side of the disk differ usually). For example, > one of my disks gives: > > /dev/hda1: > Timing buffer-cache reads: 128 MB in 0.80 seconds =160.00 MB/sec > Timing buffered disk reads: 64 MB in 2.88 seconds = 22.22 MB/sec > > /dev/hda4: > Timing buffer-cache reads: 128 MB in 0.81 seconds =158.02 MB/sec > Timing buffered disk reads: 64 MB in 3.94 seconds = 16.24 MB/sec > > You can find out by measuring the underlying partitions directly or > eliminate that factor by creating the RAID0 and RAID1 partitions on > the same underlying partition one after the other and running the > tests. > > > - AFAIK, RAID0 (striping) can only (mainly) benefit from 2 disks, if > the read size is larger than the size of the stripes per underlying > disk. If hdparm uses small blocks to read, RAID0 will (or better: > should ;) only be marginally faster than the underlying disk. You > see, it is read from one, one, one, one, two, two, two, two, one, > one, one, one and so on. > > If the block read is larger than the stripe size, it will be read > one/two, one/two, one/two, which is what usually gives the speed > improvement on reading large files. You also get a nice boost with > disk seeks and so on, but > > Note that if the assumption is true, this (the test reading small > blocks in row) is an uncommon case: usually either you read small > files from not-adjoining blocks, then you will gain from disk seeks > getting distributed to two disks, or you read large files, where you > gain from (almost) parallel reads. Neither would be the case with > small test blocks. > > > RAID1 (mirroring) on the other hand, may distribute reads > arbitrarily to the two underlying disks, because they both contain > the same data and therefore is not affected by such patterns. > > Note that the above is only a possibility based on the behaviour of > RAID0 and RAID1. I do not know if the above really matters in this > case. If you want to know, you can try to play with the size for > striping and then run the test again. > > > Bye, > > Benjamin. > > > > Now I have here a /dev/md0 which is a Raid1 and /dev/md4 which is Raid0 > > > > now using hdparm to test the speed.. > > > > # hdparm -Tt /dev/md0 > > /dev/md0: > > Timing buffer-cache reads: 128 MB in 0.52 seconds =246.15 MB/sec > > Timing buffered disk reads: 64 MB in 1.60 seconds = 40.00 MB/sec > > > > # hdparm -Tt /dev/md4 > > /dev/md4: > > Timing buffer-cache reads: 128 MB in 0.51 seconds =250.98 MB/sec > > Timing buffered disk reads: 64 MB in 2.58 seconds = 24.81 MB/sec > > > [...]
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