On 11 September 2014 06:59, NeilBrown <neilb@xxxxxxx> wrote: > On Thu, 11 Sep 2014 06:48:31 +0200 Bostjan Skufca <bostjan@xxxxxx> wrote: > >> On 11 September 2014 02:31, NeilBrown <neilb@xxxxxxx> wrote: >> > On Wed, 10 Sep 2014 23:24:11 +0200 Bostjan Skufca <bostjan@xxxxxx> wrote: >> >> What does "properly" actually mean? >> >> I was doing some benchmarks with various raid configurations and >> >> figured out that the order of devices submitted to creation command is >> >> significant. It also makes raid10 created in such mode reliable or >> >> unreliable to a device failure (not partition failure, device failure, >> >> which means that two raid underlying devices fail at once). >> > >> > I don't think you've really explained what "properly" means. How exactly do >> > you get better throughput? >> > >> > If you want double-speed single-thread throughput on 2 devices, then create a >> > 2-device RAID10 with "--layout=f2". >> >> I went and retested a few things and I see I must have done something >> wrong before: >> - regardless whether I use --layout flag or not, and >> - regardless of device cli arg order at array creation time, >> = I always get double-speed single-thread throughput. Yaay! >> >> Anyway, the thing is that regardless of -using -layout=f2 or not, >> redundancy STILL depends on the order of command line arguments passed >> to mdadm --create. >> If I do: >> - "sda1 sdb1 sda2 sdb2" - redundandcy is ok >> - "sda1 sda2 sdb1 sdb2" - redundancy fails >> >> Is there a flag that ensures redundancy in this particular case? >> If not, don't you think the naive user (me, for example) would assume >> that code is smart enough to ensure basic redundancy, if there are at >> least two devices available? > > I cannot guess what other people will assume. I certainly cannot guard > against all possible incorrect assumptions. > > If you create an array which doesn't have true redundancy you will get a > message from the kernel saying: > > %s: WARNING: %s appears to be on the same physical disk as %s. > True protection against single-disk failure might be compromised. > > Maybe mdadm could produce a similar message... I've seen it. Kernel produces this message in both cases. >> Because, if someone wants only performance and no redundancy, they >> will look no further than raid 0. But raid10 strongly hints at >> redundancy being incorporated in it. (I admit this is anecdotal, based >> on my own experience and thought flow.) > > I really don't think there is any value is splitting a device into multiple > partitions and putting more than one partition per device into an array. > Have you tried using just one partition per device, making a RAID10 with > --layout=f2 ?? Yep, I tried raid10 on 4 devices with layout=f2, it works as expected. No problem there. And I know it is better if you have 4 devices for raid10, you are right there. That is the expected use case. But if you only have 2, you are limited to the options with those two. Now, if I create raid1 on those two, I get bad single-threaded read performance. This usually does not happen with hardware RAIDs. This is the reason I started looking into posibility of using multiple partitions per disk, to get something which reads off both disks even for single "client". Raid10 seemed an option, and it works, albeit a bit hackish ATM. This is also the reason I asked for code locations, to look at it and maybe send in patches for review which make a bit more inteligent data-placement guesses in the case mentioned above. Would this be an option of interest to actually pull it it? b. -- 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
