61 lines
2.4 KiB
Plaintext
61 lines
2.4 KiB
Plaintext
---
|
|
layout: default
|
|
title: Directly Attache Storage with redundancy and integrity using LVM
|
|
date: 2023-06-24 20:11 +0200
|
|
tags: LVM
|
|
---
|
|
Directly Attached Storage (DAS) can be effectively used for frequent backups.
|
|
Risk of data loss can be prevented by means of redundancy and integrity check
|
|
mechanisms.
|
|
|
|
Data on disk can be lost due to:
|
|
* hardware error correction insufficiency or complete disk failure - which is
|
|
obvious and can be remedied using multiple copies of same data on multiple
|
|
disks (RAID). RAID for Logical Volumes is provided by @dm-raid@ kernel module
|
|
and offers functionality similar to @mdadm@ utility,
|
|
|
|
* silent data corruption - when erros slip through disk's error detection
|
|
mechanisms and remain undetected. Risk can be considerably reduced by using
|
|
checksums of data stored on additional integrity sub-LVs using @dm-integrity@
|
|
module.
|
|
|
|
RAID and integrity can be combined: each RAID sub-LV has its per-sector
|
|
checksums stored on integrity sub-LVs. Whenever checksum mismatch detects silent
|
|
data corruption, restoration from uncorrupted source is possible thanks to disk
|
|
redundancy. As of LVM tools 2.03.21 (2023-04-21), detecting which disk holds
|
|
uncorrputed data would be impossible without @dm-integrity@. Even if
|
|
3-disk-RAID1 or RAID6 is used - which theoretically is enough to find source of
|
|
single sector corruption event - the mechanisms are not implemented (see: _man
|
|
lvmraid, "Scrubbing Limitations"_).
|
|
|
|
|
|
h3. Setup
|
|
|
|
First create RAID1, then extend it with integrity layer. Integrity block size
|
|
cannot be smaller than drive's logical sector size and preferably should match
|
|
file system block size. Even before sync is finished, filesystem can be created
|
|
on LV:
|
|
|
|
{% highlight bash %}
|
|
# lvcreate --type raid1 --mirrors 1 -n backup -L 4t vgbackup /dev/sda /dev/sdb
|
|
# cat /sys/class/block/nvme0n1/queue/logical_block_size
|
|
512
|
|
# dumpe2fs -h /dev/vgbackup/backup | grep 'Block size'
|
|
Block size: 4096
|
|
# lvconvert --raidintegrity y --raidintegrityblocksize 4096 --raidintegritymode bitmap /dev/vgbackup/backup
|
|
# mkfs.ext4 -b 4096 -O ^has_journal /dev/vgbackup/backup
|
|
{% endhighlight %}
|
|
|
|
Both operations - creating RAID mirror and adding integrity layer - require
|
|
initial sync, the progress of which can be monitored:
|
|
|
|
{% highlight bash %}
|
|
lvs -a -o name,segtype,devices,sync_percent
|
|
{% endhighlight %}
|
|
|
|
Drives can be detached for later synchronization at any time:
|
|
|
|
{% highlight bash %}
|
|
vgchange -an vgbackup
|
|
{% endhighlight %}
|