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disks.md

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Automate partitioning a disk

sgdisk

  • erase partition table sgdisk -Z disk
  • print partition table sgdisk -p disk
  • create partition 1 of size 50k: sgdisk --new 1::+50K disk

sfdisk

  • create a GPT partition table with one partition of size 500M and the 2nd partition taking the rest of available space 
    sfdisk image.img <<EOF
label: gpt

EFI-partition: size=+500M
+
EOF

Remove/restore a disk

  • echo 1 > /sys/block/sdX/device/delete
  • for host in $(ls /sys/class/scsi_host/); do echo "- - -" > /sys/class/scsi_host/${host}/scan; done

Working with blktrace

  • online monitoring blktrace -d /dev/sda -o - | blkparse -i -

Otherwise, blktrace -d /dev/sda gonna write to sda.blktrace.* files, which can later be read with blkparse sda or by using an exact file blkparse sda.blkatrace.0.

Reading blkparse output

It's written in man, paragraph DEFAULT OUTPUT, but the man is a bit confusing: part of the output, where process name and offsets, is mentioned as some "default header" (even though it's in the end of each line, rarely a place for headers).

Offsets are written in format like 1234567 + 16, where the left part is the block index and the right one is the number of blocks being acted upon. A block is usually 512 bytes. The offset is apparently in decimals since I've never seen an abcdef in it.

Mapping blktrace/blkparse offset to an actual file

As of writing the words, blkparse shows global indices even if you pointed blktrace at a partition. So:

  1. Convert block index to an offset by multiplying by 512
  2. Substract offset of the beginning of the partition (I found it out by using gdisk -l /dev/sdX to get partition "start sector", and then multiplied it by 512)

What to do next varies for file systems. There's an answer for ext-family using debugfs. For btrfs you translate that to logical address with this script and then use it with btrfs inspect-internal, for example: sudo btrfs inspect-internal logical-resolve 378331914240 /.

Misc

  • block size is usually 512 bytes. AFAIR it was hardcoded. Anyway, it can be gotten with blockdev --report /dev/sdX
  • gdisk shows offsets in sectors, and the sector size is usually 512. As a hint that this is the case note whether first partitions starts at 2048 offset. 2048 * 512 = 1MB.
  • the amount of disk IO may be monitored through means of /proc/diskstats, by checking its content before and after doing some IO. The content isn't human-readable, but this script may help. Worth noting though, it seems to not work with brd module emulating disks in RAM. The read/write count always stays zero for it, probably worth reporting a bug.
  • ALUA disk is a virtual devices that is used to access another actual device. The important point is that the device is accessible both as a non-virtual device and a virtual one.

RAID misc

duplicating mount points

I noted it with MD RAID, but may be not specific to it. Suppose you have a RAID over sdX and sdY. Issuing lsblk -f may give something like

sdX
    md0p0 mount-pointX
sdY
    md0p0 mount-pointX

Note the mount point is the same! Devices are different, but mount point is the same. This is actually just a confusing output: you only have one md0p0 device, and hence one mount point. But the device itself is created over two physical devices sdX and sdY.