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-   -   LVM mirror also mirrored log, why? (

sluge 04-10-2012 03:50 AM

LVM mirror also mirrored log, why?
I found that LVM has a new feature, it can mirror not only data, but log also. Why? Is log data is so important? It will be regenerated if disk fails.

MensaWater 04-10-2012 08:54 AM

At a guess: Performance. If you have data important enough to mirror it may be it is also important enough that you don't want to wasted time in regenerating log info to get the LVs back online.

Satyaveer Arya 04-10-2012 02:39 PM

LVM mirroring at a minimum requires 3 physical volumes.

2 volumes for the both sides of the mirror and a volume for log.

The way mirroring works in LVM is that the system creates arbitrary data blocks(e.g. 512 KB in size) and it's these individual blocks that are kept in sync across the drives. The log is the essential mechanism for the system to keep the data of sync status in across reboots, hence it can't be on any of the mirrored drives. You can also configure the system to keep the log in the RAM, but this means that your drives will have to be synced every time your system's rebooted (which's a VERY bad idea, because syncing is among the slowest LVM/RAID operations ever).

sluge 04-11-2012 12:52 AM

[QUOTE=Satyaveer Arya;4649508]LVM mirroring at a minimum requires 3 physical volumes.

In RHEL you can use at minimum 2 PVs

Satyaveer Arya 04-11-2012 12:39 PM

Exactly you can take 2 PVs, it will work but in my last post I also mentioned about the another PV i.e., for log.

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