On Wed, 2005-08-24 at 15:43 +0200, Mirko Benz wrote: > Hello, > > The RAID5 configuration is: 8 SATA disks, 8 port Marvel SATA PCI-X > controller chip (SuperMicro board), Dual Xeon, 1 GB RAM, stripe size > 64K, no spare disk. u have good luck on this Marvel SATA. When I use it with supermicro, I got very bad performance even with RAID0. > > Measurements are performed to the ram md device with: > disktest -PT -T30 -h1 -K8 -B65536 -ID /dev/md0 > using the default stripe size (64K). 128K stripe size does not make a > real difference. > when u run this command, also run "iostat 1" at another console and see how many read and write. also run "time dd if=/dev/zero of=/dev/mdX bs=1M" and see speed u get and see what iostat tell u. > We have also increased the RAID 5 stripe cache by setting NR_STRIPES to > a larger value but without any perceptible effect. > > If Linux uses "stripe write" why is it so much slower than HW Raid? Is > it disabled by default? > > 8 disks: 7 data disks + parity @ 64k stripe size = 448k data per stripe > The request size was smaller (tested up to 256K) than the size of a stripe. > We have seen errors for larger request sizes (e.g. 1 MB). Does Linux > require the request size to be larger than a stripe to take advantage > of "stripe write"? > > Regards, > Mirko > > Ming Zhang schrieb: > > >On Wed, 2005-08-24 at 10:24 +0200, Mirko Benz wrote: > > > > > >>Hello, > >> > >>We have recently tested Linux 2.6.12 SW RAID versus HW Raid. For SW Raid > >>we used Linux 2.6.12 with 8 Seagate SATA NCQ disks no spare on a Dual > >>Xeon platform. For HW Raid we used a Arc-1120 SATA Raid controller and a > >>Fibre Channel Raid System (Dual 2 Gb, Infortrend). > >> > >>READ SW:877 ARC:693 IFT:366 > >>(MB/s @64k BS using disktest with raw device) > >> > >>Read SW Raid performance is better than HW Raid. The FC RAID is limited > >>by the interface. > >> > >>WRITE SW:140 ARC:371 IFT:352 > >> > >>For SW RAID 5 we needed to adjust the scheduling policy. By default we > >>got only 60 MB/s. SW RAID 0 write performance @64k is 522 MB/s. > >> > >> > >how u test and get these number? > > > >what is u raid5 configuration? chunk size? > > > > > > > >>Based on the performance numbers it looks like Linux SW RAID reads every > >>data element of a stripe + parity in parallel, performs xor operations > >>and than writes the data back to disk in parallel. > >> > >>The HW Raid controllers seem to be a bit smarter in this regard. When > >>they encounter a large write with enough data for a full stripe they > >>seem to spare the read and perform only the xor + write in parallel. > >>Hence no seek is required and in can be closer to RAID0 write performance. > >> > >> > >this is stripe write and linux MD have this. > > > > > > > > > > > >>We have an application were large amounts of data need to be > >>sequentially written to disk (e.g. 100 MB at once). The storage system > >>has an USV so write caching can be utilized. > >> > >>I would like to have an advice if write performance similar to HW Raid > >>controllers is possible with Linux or if there is something else that we > >>could apply. > >> > >>Thanks in advance, > >>Mirko > >> > >>- > >>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
