On Thu, Feb 03, 2011 at 01:50:49PM -0200, Roberto Spadim wrote: > hummm, nice > keld (or anyone), do you know someone (with time, not much, total time > i think it?s just 2 hours) to try develop modifications on raid1 > read_balance function? maybe our very productive Polish friends at Intel could have a look. But then again, I am not sure it is productive. I think raid1 is OK, You could have a look at raid10, where "offset" has been discussed as being the better layout for ssd. > what modification, today read_balance have distance (current_head - > next_head), multiply it by a number at /sys/block/md0/distance_rate, > and make add read_size*byte_rate (byte_rate at > /sys/block/md0/byte_read_rate), with this, the algorithm will make > minimal time, and not minimal distance > with this, i can get better read_balance (for ssd) > for a second time we could implement device queue time to end (i think > we will work about 1 day to get it working with all device > schedulers), but it?s not for now Hmm, I thought you wanted to write new elevator schedulers? best regards keld > > 2011/2/3 Keld Jørn Simonsen <keld@xxxxxxxxxx>: > > On Thu, Feb 03, 2011 at 12:35:52PM -0200, Roberto Spadim wrote: > >> =] i think that we can end discussion and conclude that context (test > >> / production) allow or don't allow lucky on probability, what's lucky? > >> for production, lucky = poor disk, for production we don't allow > >> failed disks, we have smart to predict, and when a disk fail we change > >> many disks to prevent another disk fail > >> > >> could we update our raid wiki with some informations about this discussion? > > > > I would like to, but it is a bit complicated. > > Anyway I think there already is something there on the wiki. > > And then, for one of the most important raid types in Linux MD, > > namely raid10, I am not sure what to write. It could be raid1+0, or > > raid0+1 like, and as far as I kow, it is raid0+1 for F2:-( > > but I don't know for n2 and o2. > > > > The German version on raid at wikipedia has a lot of info on probability > > http://de.wikipedia.org/wiki/RAID - but it is wrong a number of places. > > I have tried to correct it, but the German version is moderated, and > > they don't know what they are writing about. at least in some places, refusing to correct errors. > > http://de.wikipedia.org/wiki/RAID > > > > Best regards > > Keld > > > >> 2011/2/3 Drew <drew.kay@xxxxxxxxx>: > >> >> for test, raid1 and after raid0 have better probability to don't stop > >> >> raid10, but it's a probability... don't believe in lucky, since it's > >> >> just for test, not production, it doesn't matter... > >> >> > >> >> what i whould implement? for production? anyone, if a disk fail, all > >> >> array should be replaced (if without money replace disk with small > >> >> life) > >> > > >> > A lot of this discussion about failure rates and probabilities is > >> > academic. There are assumptions about each disk having it's own > >> > independent failure probability, which if that can not be predicted > >> > must be assumed to be 50%. At the end of the day I agree that when > >> > the first disk fails the RAID is degraded and one *must* take steps to > >> > remedy that. This discussion is more about why RAID 10 (1+0) is better > >> > then 0+1. > >> > > >> > On our production systems we work with our vendor to ensure the > >> > individual drives we get aren't from the same batch/production run, > >> > thereby mitigating some issues around flaws in specific batches. We > >> > keep spare drives on hand for all three RAID arrays, so as to minimize > >> > the time we're operating in a degraded state. All data on RAID arrays > >> > is backed up nightly to storage which is then mirrored off-site. > >> > > >> > At the end of the day our decision around what RAID type (10/5/6) to > >> > use was based on a balance between performance, safety, & capacity > >> > then on specific failure criteria. RAID 10 backs the iSCSI LUN that > >> > our VMware cluster uses for the individual OSes, and the data > >> > partition for the accounting database server. RAID 5 backs the > >> > partitions we store user data one. And RAID 6 backs the NASes we use > >> > for our backup system. > >> > > >> > RAID 10 was chosen for performance reasons. It doesn't have to > >> > calculate parity on every write so for the OS & database, which do a > >> > lot of small reads & writes, it's faster. For user disks we went with > >> > RAID 5 because we get more space in the array at a small performance > >> > penalty, which is fine as the users have to access the file server > >> > over the LAN and the bottle neck is the pipe between the switch & the > >> > VM, not between the iSCSI SAN & the server. For backups we went with > >> > RAID 6 because the performance & storage penalties for the array were > >> > outweighed by the need for maximum safety. > >> > > >> > > >> > > >> > -- > >> > Drew > >> > > >> > "Nothing in life is to be feared. It is only to be understood." > >> > --Marie Curie > >> > > >> > > >> > >> > >> > >> -- > >> Roberto Spadim > >> Spadim Technology / SPAEmpresarial > >> -- > >> 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 > > > > > > -- > Roberto Spadim > Spadim Technology / SPAEmpresarial -- 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
