bitmap status question

bitmap status question

[David Greaves]

How do I interpret:
      bitmap: 0/117 pages [0KB], 1024KB chunk
in the mdstat output

what does it mean when it's, eg: 23/117

cu:/huge# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [raid4] [raid6]
md0 : active raid5 sdf1[5] sde1[4] sdd1[3] sdc1[2] sdb1[1] sda1[0]
hdb1[6](S)
      1225543680 blocks level 5, 4096k chunk, algorithm 2 [6/6] [UUUUUU]
      bitmap: 0/117 pages [0KB], 1024KB chunk


David

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Re: bitmap status question

[Paul Clements]

David Greaves wrote:
> How do I interpret:
>       bitmap: 0/117 pages [0KB], 1024KB chunk
> in the mdstat output
> 
> what does it mean when it's, eg: 23/117

This refers to the in-memory bitmap (basically a cache of what's in the 
on-disk bitmap -- it allows bitmap operations to be more efficient).

If it's 23/117 that means there are 23 of 117 pages allocated in the 
in-memory bitmap. The pages are allocated on demand, and get freed when 
they're empty (all zeroes). The in-memory bitmap uses 16 bits for each 
bitmap chunk to count all ongoing writes to the chunk, so it's actually 
up to 16 times larger than the on-disk bitmap.

--
Paul