Re: iostat with raid device...

[Linux Raid Study <linuxraid.study@gmail.com>]

If I use --assume-clean in mdadm, I see performance is 10-15% lower as
compared to the case wherein this option is not specified. When I run
without --assume_clean, I wait until mdadm prints "recovery_done" and
then run IO benchmarks...

Is perf drop expected?

Thanks.

On Mon, Apr 11, 2011 at 3:18 AM, NeilBrown <neilb@suse.de> wrote:
> On Mon, 11 Apr 2011 10:53:55 +0100 Robin Hill <robin@robinhill.me.uk> 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@robinhill.me.uk> 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@robinhill.me.uk> 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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