FW: the mirror log is NOT kept on disk

FW: the mirror log is NOT kept on disk

[Eli Malul]

[-- Attachment #1.1: Type: text/plain, Size: 1281 bytes --]

I just saw that clean_bits is being saved to the disk and is used to
update sync_bits after crash/reboot (although the default behavior is to
reset all the bits hence, resync is done for each resume).

 

But I still do not understand how should I create a mapped device with
dmsetup tool which wouldl survive crash/reboot.

 

Can anyone help?

 

From: Eli Malul 
Sent: Sunday, March 20, 2011 6:19 PM
To: 'dm-devel@redhat.com'
Subject: the mirror log is NOT kept on disk

 

Hi,

 

I tried a small test where I suspended and resumed a mirrored device
without IO going on.

My expectation was that the mirror set will not get re-synced but
unfortunately it did.

 

According the documentation the mirror log is kept on disk when creating
a mirror logical device with 'disk' log type as an input parameter.

I looked at the code and I didn't see where the sync_bits are being
written to the disk or read from the disk but only the number of regions
is being saved.

 

How should a mirror device survive reboots/crashes without syncing the
entire mirror set?

 

BTW - another problem is that after reboot the created logical device
disappears, is there an easy way to keep it persistent or should I
create at each boot?

 


[-- Attachment #1.2: Type: text/html, Size: 5346 bytes --]

[-- Attachment #2: Type: text/plain, Size: 0 bytes --]

Re: FW: the mirror log is NOT kept on disk

[Milan Broz]

On 03/21/2011 02:10 PM, Eli Malul wrote:

> But I still do not understand how should I create a mapped device
> with dmsetup tool which wouldl survive crash/reboot.

You cannot do that without implementing persistent metadata handling
yourself.

Use LVM here. dmsetup is simple wrapper to DM-ioctl, low level utility.

LVM is based on top of that and solves all the problems with persistent
metadata you just found.

Milan