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Martin Hope
Brian
Asked: 2024-12-01 02:46:14 +0800 CST
  • 7

I have a 2 disk MD raid1 array. One of the drives is slow so I want to replace it. But for paranoia[1] and safety's sake, I want to add the new (3rd) drive and sync it before removing the slow drive, since, well, it's just slow, but otherwise is working and has valid data on it and I want to avoid degrading the array before I have to.

So I've added the third disk and it's started syncing, so all is good so far. But it's only using 1 of the existing 2 drives as the source of the sync, and wouldn't you know it according to what I call the 50/50/90 rule[2], it's using the slow disk as the (only) source of the sync. I can see that it's (only) using the slow disk using sar:

01:21:55 PM       tps     rkB/s     wkB/s     dkB/s   areq-sz    aqu-sz     await     %util DEV
01:22:00 PM     91.60      0.00  45147.30      0.00    492.87      0.14      1.48     11.26 sdb
01:22:00 PM      0.40      0.00      1.70      0.00      4.25      0.13    314.00     48.08 sde
01:22:00 PM     91.60  45145.60      1.70      0.00    492.87      3.41     37.26     88.30 sdd
01:22:00 PM      0.00      0.00      0.00      0.00      0.00      0.00      0.00      0.00 md0

sdd is the slow disk. If MD were reading from sde instead, the speed would be about 4x that:

# dd if=/dev/sde of=/dev/null bs=1M count=5000
5000+0 records in
5000+0 records out
5242880000 bytes (5.2 GB, 4.9 GiB) copied, 23.1946 s, 226 MB/s

Frankly I am surprised that MD is being that simplistic and that it doesn't read from all of the disks when rebuilding, for exactly this kind of reason/case.

So, is there any way I can force the use of sde in this sync, either in addition to sdd or instead of it?

P.S. The sync speed is not being limited by dev.raid.speed_limit_max:

# sysctl -n dev.raid.speed_limit_max
20000000

[1] Call me paranoid, but you know the moment that your array is down to a single disk is when that single disk is going fail.

[2] If you have a 50/50 chance of something happening one way or the other, 90% of the time it will be the way you didn't want.

mdadm
Martin Hope
Brian
Asked: 2018-06-07 03:55:51 +0800 CST
  • 1

I have an entry in my routing table on my Linux router that I didn't add:

prohibit 2604:[redacted]:d772 from :: dev lo  proto kernel  src 2607:[redacted]::1  metric 4294967295  error -13 pref medium

and I can't seem to remove:

# ip -6 route del prohibit 2604:[redacted]:d772
RTNETLINK answers: No such process
# ip -6 route del prohibit 2604:[redacted]:d772 from :: dev lo  proto kernel  src 2607:[redacted]::1
RTNETLINK answers: No such process

So as much as I want to remove that "prohibit" route (feed a man a fish), I'd also like to know how it got added (teach a man to fish) since I know I didn't add it and it's not part of my configuration.

linux