On 11/02/2015 05:01 PM, Heinz Mauelshagen wrote:
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.
Ah ok. I was confused because what dm-raid calls a chunk, mdraid calls a
stripe, and what dm-raid calls a region, bitmap.c calls a chunk :-)
<snip example md size calculation>
I added some explanation to your size calculation and formatted it to be
suitable to add to Documentation/device-mapper/dm-raid.txt:
The Metadata Device
-------------------
While users of dm-raid need not parse or access the contents of the
metadata device, they must still ensure it is of a certain minimum
size. Fixed-size metadata consumes 8KiB, and then one extra bit must
be allocated for each region in the raid device.
For example:
With a 64KiB region size and 1TiB raid1 set size (1TiB*Bit / 64 KiB =
16777216 Bit; 16777216 Bit / (8 Bit/Byte) / (1024 Byte/KiB) = 2048 KiB
8 KiB fixed-size metadata
+ 2048 KiB region bitmap
-------------------------
2056 KiB total
A device must contain fewer than 2^21 regions. With the default region
size of 4MiB this limit is reached at 8TiB. Therefore, raid devices
larger than that will need to use a larger region size, set with the
"region_size" parameter.
(end)
What do you think?
Anything else to add or change in dm-raid.txt while we're at it?
Thanks -- Regards -- Andy
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