Hi,
Here, I got two confusing questions about Linux MD:
I. Why when initial create RAID5, mdadm marks a physical disk as "spare"?
Is this for random write with RMW, or for "sync" speed?
II. The write intent bitmap, each disk in RAID with a "write intent
bitmap", or the whole RAID with one "write intent bitmap"?
If the whole RAID with one "write intent bitmap", how to know
which disk's data need reconstruct, or just use the data disks'
data to calculate the P data, and write to the P disk? If the only
one "write intent bitmap", how to decide which disk to save
the "write intent bitmap"?
And is there has any MD design architecture document?
Thanks a lot
I) Can you give the complete mdadm command used to create it ?
Normally it should create a RAID5 without spares. (unless instructed otherwise/you passed the wrong options)
Also giving us the output of mdadm --detail /dev/mdXXX could help
II) ***Disclaimer*** following information below might not be accurate, but such a system could work.
If it's incorrect it should help you understand when someone corrects me.
mdadm --examine /dev/sdXX shows me "Internal Bitmap : 8 sectors from superblock"
This would indicate there is a bitmap on each drive (although I'm not sure, theoretically you could RAID it, but why increase complexity).
However the RAID only need 1 write indent map.
But in the worst case scenario only 1 disk is left, so a copy is maintained on each drive.
Example:
Write indent map for 512K disk using 64K chunks
Bit 1: Synchronized
Bit 0: Not synced
| Chunk | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Bit | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
When you write in Chunk 1, the bit is set to 0.
Now assume 1 of the disk power connector flies of, and the write to the chunk fails.
| Chunk | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Bit | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
Meanwhile another write is done to Chunk 2, new bitmap:
| Chunk | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Bit | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 |
Now when you plug the disk back in it looks for unwritten chunks, and it find 1 and 2, now it nows it can start from this.
(Note it reject the bitmap of the disk you plugged back in.)
In case you are building a new raid something simular occurs:
This would be the start bitmap:
| Chunk | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Bit | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
As each chunk is sycned the bit is set to 1:
C1234578
B0000000 Later it becomes:
B1000000 Then later it becomes
B1100000 ...
So at any point you can reboot, and the raid will know where to continue by looking at the non-sycned bitmaps.
Also see the wiki: https://raid.wiki.kernel.org/index.php/Write-intent_bitmap
Killian De Volder
On 18-07-14 16:21, Henry Cai wrote:
Hi,
Here, I got two confusing questions about Linux MD:
I. Why when initial create RAID5, mdadm marks a physical disk as "spare"?
Is this for random write with RMW, or for "sync" speed?
II. The write intent bitmap, each disk in RAID with a "write intent
bitmap", or the whole RAID with one "write intent bitmap"?
If the whole RAID with one "write intent bitmap", how to know
which disk's data need reconstruct, or just use the data disks'
data to calculate the P data, and write to the P disk? If the only
one "write intent bitmap", how to decide which disk to save
the "write intent bitmap"?
And is there has any MD design architecture document?
Thanks a lot
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