Hi,
I'm currently upgrading a NAS system with new disks. Since I'm
changing the filesystem type and due to a lack of enough SATA ports I
have to do add one new disk at a time, copy data, shrink the old
filesystem, remove an old disks and repeat. I've started with a 2 disk
raid5, copied data, freed a 3rd SATA slot and added the 3rd new disk.
Now I'm reshaping the new raid5 from 2 disks to 3 disks:
md0 : active raid5 sdd1[3] sdc1[2] sda1[0]
3907015168 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]
[==>..................] reshape = 14.0% (547848840/3907015168) finish=1355.4min speed=41302K/sec
so far everything works fine. But the speed is rather low and the IO
traffic is higher than I think it should be:
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 604.33 81706.00 40904.73 4902360 2454284
md0 0.00 0.00 0.00 0 0
sdc 440.78 81839.40 40542.07 4910364 2432524
sdd 509.72 0.00 40817.67 0 2449060
To reshape the kernel needs to read 1 data block from sda, 1 data
block from sdc, compute the XOR of both blocks and write 2 data blocks
+ parity block back to the 3 disks. The kernel read 160MB/s, add
80MB/s parity and it should write 240MB/s (or 80MB/s per disk).
Instead it only writes 120MB/s (40MB/s per disk), only half of what I
expect.
So what is going on there? Is the kernel reading both data and parity
blocks and verifying them?
MfG
Goswin
PS: please CC me on replies.
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