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_posts/sysadm/2023-06-24-directly-attache-storage-with-redundancy-and-integrity-using-lvm-lvmraid.tl

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+---
+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. 
+
+{% highlight bash %}
+lvcreate --type raid1 --mirrors 1 -n backup -L 4t vgbackup /dev/sda /dev/sdb
+{% endhighlight %}
+
+Integrity block size cannot be smaller than drive's logical sector size and
+preferably should match file system block size.
+
+{% highlight bash %}
+$ 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
+{% 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 %}
+
+Even before sync is finished, filesystem can be created on LV:
+
+{% highlight bash %}
+mkfs.ext4 -b 4096 -O ^has_journal /dev/vgbackup/backup
+{% endhighlight %}
+
+and the drives can be detached for later synchronization:
+
+{% highlight bash %}
+vgchange -an vgbackup
+{% endhighlight %}