FW: the mirror log is NOT kept on disk
 

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).
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But I still do not understand how should I create a mapped device with dmsetup tool which wouldl survive crash/reboot.
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Can anyone help?
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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
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Hi,
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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.
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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.
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How should a mirror device survive reboots/crashes without syncing the entire mirror set?
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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?
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FW: the mirror log is NOT kept on disk
 

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

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