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Re: [linux-lvm] LVM 0.8 and reiser filesystem



On Wed, Jun 07, 2000 at 06:04:55PM +0200, Jos Visser wrote:
> The way HP's logical volume manager does it is by maintaining a kind of 
> data log somewhere in the volume metadata.  This log (let's call it the 
> Mirror Write Cache) is effectively a bitmap which keeps track of which 
> blocks in the logical volume are hit by a write.  The unit of 
> granularity here is not an individual block, but something that is 
> called a Large Track Group (LTG, let's say a couple of MB).  Whenever 
> all parallel writes are finished, the corresponding LTG bit in the MWC 
> is cleared and the MWC on disk is (eventually) updated.
> 
> After a crash when the Volume Group is activated, all copies (plexes)
> of a volume must be synchronized. The VM software inspects the MWC, and
> then knows which blocks might be out of sync across the plexes. Only
> these blocks are then synchronized using a read from the preferred plex
> and write to all other plexes. The MWC is used to prevent a full sync
> after a crash.

Sounds clever. I really wish Linux raid would use this optimization :-) 
(The slowness of raidcheck is a big problem) 

-Andi



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