RE: "bitmap" line in /proc/mdstat

["Peter Sangas" <pete@wnsdev.com>]

Can you quantify the performance decrease and if rebuild time is not as important as performance how does one remove the bitmap?

Thank you,
Peter

-----Original Message-----
From: Phil Turmel [mailto:philip@turmel.org] 
Sent: Friday, October 14, 2016 4:53 AM
To: Ramon Hofer; linux-raid@vger.kernel.org
Subject: Re: "bitmap" line in /proc/mdstat

On 10/14/2016 06:06 AM, Ramon Hofer wrote:

> From what I was reading [1,2] it is useful for faster rebuilds.
> But why is it not present for md[1234] as I used the same command to 
> create these RAIDs?
> Is it only possible for the last of the linear devices, or is it only 
> used for almost empty devices, or was the bitmap introduced in a 
> recent update?

Bitmaps have been available for a long time, but including them on a new array by default is recent.  They are most useful if a device is unexpectedly disconnected but otherwise OK.  mdadm --re-add will allow that device to rejoin the array and only the missing data (tracked by the bitmap) will have to be written to sync it up.

> I wonder if I should and could add a bitmap for the other devices as well?

There is a performance hit, so it should be considered on a case by case basis.

Phil
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