| From: | Ragnar Kjørstad <postgres(at)ragnark(dot)vestdata(dot)no> |
|---|---|
| To: | Matt Clark <matt(at)ymogen(dot)net> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Peter Eisentraut <peter_e(at)gmx(dot)net>, Murthy Kambhampaty <murthy(dot)kambhampaty(at)goeci(dot)com>, pgsql-admin(at)postgresql(dot)org |
| Subject: | Re: LVM snapshots |
| Date: | 2003-03-17 15:39:31 |
| Message-ID: | 20030317153931.GD23149@vestdata.no |
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| Lists: | pgsql-admin |
On Mon, Mar 17, 2003 at 03:35:53PM -0000, Matt Clark wrote:
> > LVM-blocksizes are relatively large, 4 MB pr default, and for large
> > volumes it's common to use even bigger blocks (say 128 MB). For a 2TB
> > volume and 128 MB blocks the datastructure should take 32KB memory.
>
> 4MB is the size of a physical extent, not a block. Snapshots use 'chunks' not whole extents, and I don't know what (if any)
> relationship there is between chunks and blocks.
>
> #define LVM_SNAPSHOT_MAX_CHUNK 1024 /* 1024 KB */
> #define LVM_SNAPSHOT_DEF_CHUNK 64 /* 64 KB */
> #define LVM_SNAPSHOT_MIN_CHUNK (PAGE_SIZE/1024) /* 4 or 8 KB */
Ah, that explains it.
Sorry for adding to the confusion.
> Anyway, you definitely *don't* force LVM to copy 4MB of data the first time you touch just 1 bit on a snapshotted volume!
Probably not.
--
Ragnar Kjørstad
Zet.no
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