On 11/02/2015 08:55 PM, Andy Grover wrote:
Hi all,
In looking at md-raid one question I had was exactly how large does
the metadata device need to be in comparison to the data device? It
contains a padded-to-4Kbyte sized raid metadata struct, and then also
a write intent bitmap that is sized based upon the number of stripes,
with 16 bits per stripe.
There is no direct relationship between stripes and bitmap size, because the
bitmap holds a bit per region.
I'm restricting this to metadata kept by dm-raid (neither md/md raid1
clustered or md with external metadata),
because you seem to utilize that:
the metadata contains a 4KiB padded struct to describe the array (the
raid superblock) at offset 0
followed by 4KiB for the bitmap header. After those 2, a bit per region
(i.e. blocks / region_size) is allocated.
Say, for 64KiB region size and 1TiB raid1 set size (1TiB*Bit / 64 KiB =
16777216 Bit; 16777216 Bit / (8 Bit/Byte) / (1024 Byte/KiB) = 2048 KiB:
4KiB superblock
4KiB bitmap header
2048 KiB bitmap
-------------------------
2056KiB total
Is it correct to say the minimum size of the metadata device should be:
4096 + (2 * stripe_count)
bytes?
Thanks -- Andy
p.s. LVM2 allocates 1 extent (default 4MiB) for the raid meta device.
But it's not clear if given a large enough raid dev, and configured
with a much smaller extent size than the default, if one extent might
not be enough.
It is actually very clear that in case of small lvm2 extent and raid
region size,
one extent is easily not big enough.
That's why I am developing dynamic raid metadata allocation for lvm2.
Heinz
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