Re: Bitmap did not survive reboot

[Doug Ledford <dledford@redhat.com>]

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On 11/11/2009 11:55 PM, Leslie Rhorer wrote:
>>>     Data Offset : 272 sectors
>>>    Super Offset : 8 sectors
>>
>> The above two items are what you need for both version 1.1 and 1.2
>> superblocks in order to figure things out.  The data, aka the filesystem
>> itself, starts at the Data Offset which is 272 sectors.  The superblock
>> itself is 8 sectors in from the front of the disk because you have
>> version 1.2 superblocks.  So, 272 - 8 - size of the superblock, which is
>> only a sector or two, is how much internal space you have.  So, in your
>> case, you have about 132k of space for the bitmap.
> 
> 	OK.  The 10 drive system shows:
> 
> bitmap: 0/466 pages [0KB], 1024KB chunk
> 
> 	The 7 drive system shows:
> 
> bitmap: 0/350 pages [0KB], 2048KB chunk
> 
> So you think I should remove both and replace them with 
> 
> mdadm -G /dev/md0 --bitmap=internal --bitmap-chunk=65536
> 
> ?
> 
> 	While most of the files are large video files, there are a fair
> number which are smaller data files such as those of the IMAP server and
> Quicken.  I don't want performance to be too terrible for them, either.

Oh yeah, those chunk sizes are waaayyyy too small.  Definitely replace
them.  If it will make you feel better, you can do some performance
testing before and after to see why I say so ;-)  I would recommend
running these tests to check the performance change for yourself:

dbench -t 300 -D $mpoint --clients-per-process=4 16 | tail -19 >> $log_file
mkdir $mpoint/bonnie
chown nobody.nobody $mpoint/bonnie
bonnie++ -u nobody:nobody -d $mpoint/bonnie -f -m
RAID${lvl}-${num}Disk-${chunk}k -n 64:65536:1024:16 >>$log_file 2>/dev/null
tiotest -f 1024 -t 6 -r 1000 -d $mpoint -b 4096 >> $log_file
tiotest -f 1024 -t 6 -r 1000 -d $mpoint -b 16384 >> $log_file

Obviously, I pulled these tests out of a script I use where all these
various variables are defined.  Just replace the variables with
something sensible for accessing your array, run them, save off the
results, run again with a different chunk size, then please post the
results back here as I imagine they would be very informative.
Especially the dbench results as I think they are likely to benefit the
most from the change.  Note: dbench, bonnie++, and tiotest should all be
available in the debian repos.

-- 
Doug Ledford <dledford@redhat.com>
              GPG KeyID: CFBFF194
	      http://people.redhat.com/dledford

Infiniband specific RPMs available at
	      http://people.redhat.com/dledford/Infiniband


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