On Sat, Dec 19, 2009 at 12:30 AM, Thomas Fjellstrom <tfjellstrom@xxxxxxx> wrote: > On Fri December 18 2009, Bernd Schubert wrote: >> On Saturday 19 December 2009, Matt Tehonica wrote: >> > I have a 4 disk RAID5 using a 2048K chunk size and using XFS >> >> 4 disks is a bad idea. You should have 2^n data disks, but you have 2^1 + >> 1 = 3 data disks. As parity information are calculated in the power of >> two and blocks are written in the power of two, you probably have read >> operations, when you only want to write. >> >> > filesystem. Typical file size is about 2GB-5GB. I usually get around >> > 50MB/sec transfer speed when writting files to the array. Is this >> > typcial or is it below normal? A friend has a 20 disk RAID6 using the >> > same filesystem and chunk size and gets around 150MB/sec. Any input on >> > this?? >> >> I would remove two disks, to get 16 + 2 drives (2^4). Performance >> probably would be limited by CPU speed then. 150MB/s for 18 drives is >> also bad, this is only the performance of two single raid0 drives. > > I'd have to agree. My 5 disk raid5 array gets me 200-400MB/s, depending on > the kernel. I'm using a 512K chunk size, formatted with XFS, with 32 AGs, > and xfs_info reporting: sunit=128 swidth=512 blks (which should be > right...), and mounted with: > noatime,nodiratime,logbufs=8,allocsize=512m,largeio,swalloc > > oh, not quite 200MB/s, iozone is showing 112MB/s write, and 300MB/s read. > I'm pretty sure that has something to do with the writeback stuff though, > and aught to be improved in 2.6.32+ (I have yet to find a good time to > upgrade my server). I know I have seen the SAS card, and an initial array > handle more throughput than that when I was first testing stuff months and > months ago. It was more like 200-350 write, and 400-550 read. > > But yeah, 50MB/s is pretty bad for a raid array. The individual disks in my > array are all capable of more than that each. (Yes, I know raid5 will not > give a linear improvement when adding more drives, but it aught to be a heck > of a lot better than a decrease in performance) > >> >> Cheers, >> Bernd >> -- >> 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 >> > > > -- > Thomas Fjellstrom > tfjellstrom@xxxxxxx > -- > 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 > It is possible that in your setup memory speed could be the bottleneck; have you considered that maybe the issue is the size of your processor's cache compared to the size of the stripe? I know server chips from Intel and AMD typically have larger caches, and that in the consumer end the Intel chips typically also have more cache. It could be that your selection of stripe size is simply larger than the cache size and thus you actually notice the cost of dipping down to memory speeds when you're processor is starved for new data (or if it's really thrashed with other tasks). I'm not sure how much of an issue this is for me, since the idea size of my cache is only a little larger than it actually is, and DDR-2 might still be fast enough in a burst mode to read ahead of the end of the request. I do know that in my case performance is more than sufficent and the main concern as simply not loosing all of that data. -- 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
