On Mon, 11 Apr 2011 10:53:55 +0100 Robin Hill <robin@xxxxxxxxxxxxxxx> wrote: > On Mon Apr 11, 2011 at 02:36:50AM -0700, Linux Raid Study wrote: > > On Mon, Apr 11, 2011 at 2:25 AM, Robin Hill <robin@xxxxxxxxxxxxxxx> wrote: > > > On Mon Apr 11, 2011 at 01:32:34 -0700, Linux Raid Study wrote: > > >> On Sat, Apr 9, 2011 at 1:50 AM, Robin Hill <robin@xxxxxxxxxxxxxxx> wrote: > > >> > On Fri Apr 08, 2011 at 05:40:46PM -0700, Linux Raid Study wrote: > > >> > > > >> >> What I'm not sure of is if the device is newly formatted, would raid > > >> >> recovery happen? What else could explain difference in the first run > > >> >> of IO benchmark? > > >> >> > > >> > When an array is first created, it's created in a degraded state - this > > >> > is the simplest way to make it available to the user instantly. The > > >> > final drive(s) are then automatically rebuilt, calculating the > > >> > parity/data information as normal for recovering a drive. > > >> > > > >> Thanks. So, the uneven (unequal) distribution of Wrtie/Sec numbers in > > >> the iostat output are ok...is that correct? > > >> > > > If it hadn't completed the initial recovery, yes. If it _had_ completed > > > the initial recovery then I'd expect writes to be balanced (barring > > > any differences in hardware). > > > > > The initial recovery should normally be done during first few minutes > > .... this is a newly formatted disk so there isn't any user data > > there. So, if I run the IO benchmark after say 3-4 min of doing, I > > should be ok? > > > > mdam --create /dev/md0 --raid5.... > > mount /dev/md0 /mnt/raid > > mkfs.ext4 /mnt/raid > > > > ...wait 3-4 min > > > > run IO benchmark... > > > > Am I correct? > > > No, depending on the size of the drives, the initial recovery can take > hours or even days. For RAID5 with N drives, it needs to read the > entirity of (N-1) drives, and write the entirity of the remaining drive > (whether there's any data or not, the initial state of the drives is > unknown so parity data has to be calculated for the entire array). > > Check /proc/mdstat and wait until the array has completed resync before > running any benchmarks. or run mdadm --wait /dev/md0 or create the array with --assume-clean. But if the array is raid5, don't trust the data if a device fails: use this only for testing. NeilBrown > > Cheers, > Robin
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