---
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/sda/queue/physical_block_size
4096
# cat /sys/class/block/nvme0n1/queue/logical_block_size
512
# 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 %}