Op 12 maart 2012 15:20 heeft David Brown <david@xxxxxxxxxxxxxxx> het volgende geschreven: > On 12/03/2012 13:34, Caspar Smit wrote: >> >> Hi all, >> >> I don't know exactly which mailinglists to use for this one so I hope >> i used the right ones. >> >> I did some performance testing on a new system and found out some >> things I couldn't explain or didn't expect. >> At the end are some questions I hope to get answered to explain the >> tings i'm seeing in the test. >> >> >> For the next test I wanted to see if i could double the performance by >> striping an LV over 2 md's (so instead of using 10 disks/spindles, use >> 20 disks/spindles) >> >> So i added md1 to the VG as PV. >> >> Created a fresh LV striped across the two PV's using a 64KB stripe >> size and ran the test again. >> >> >> Now the total IO's in 10 seconds are 16x larger than before. 190464 / >> 10 = 19046,4 / 16 = 1190,4 /16 = the reported 75 IOPS above. >> So the 64KB blocks seem to be split into 4KB blocks (64 / 16 = 4) >> which results in a way larger total IO's. >> The IO's per disk seem to be in 64KB blocks still only now with a >> large MERGE figure besides it. (Now 4KB blocks are merged into 64KB >> blocks?) >> > > LVM will stripe the data between the two md's with a default stripe size of > 4K - thus the first 4K will go to md0, the second to md1, etc. This is > obviously terribly inefficient. For 8+2 raid6 with 64KB chunks, you want a > stripe size of 8x64K = 512KB when you create the logical volume. Ok, that makes sense. But if I for instance had created a 10 disk RAID5 md with a 64KB chunk size it would have been a stripe size of 9x64KB=576KB which is not possible. So I have to make sure I always create a raid5/6 md where the stripe size is a power of 2 when i want to use raid0 and/or LVM striping, correct? Caspar > > >> The performance does not double but stays the same as with 1 MD set >> only the total IO's are spread among the MD's. Each disk now does >> around 60 IOPS! >> >> I still wanted to see if I could double the performance and thought it >> might have something to do with LVM striping so i ditched LVM and >> created a RAID0 (md6) over md0 and md1 with a chunk size of 64KB >> again. >> > > Similarly here, you want your chunk sizes to fit a stripe on the raid > devices, not to fit the underlying devices. So try with a chunk size of > 512KB (or higher). > > mvh., > > David > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
