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Jai
Asked: 2016-06-14 03:26:10 +0800 CST

Thin LVM reducing the metadata size and performance

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I have recently created a Thin LVM. I notice the Data is at 1.60% and Meta at 1.21%.

[root@srv ~]# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
lv1 volgrp1 twi-aotz-- 410.00g 1.60 1.21

I have three questions:

  1. Is this Meta size expected. (almost equivalent to Data)
  2. Is it recommended to reduce/set metadata to a smaller value (using --poolmetadatasize).
  3. Are there any performance impact / concerns by setting a lower metadata size.

I appreciate your kind advice on this.

linux lvm centos7

1 Answers

  1. Most of the questions should be answered in lvmthin(7)

    1. Is this Meta size expected. (almost equivalent to Data)

      TL;DR: Yes.

      Both data and metadata can be automatically resized when needed (enable this in lvm.conf - see thin_pool_autoextend_threshold and thin_pool_autoextend_percent). The default size of metadata is calculated by lvcreate to be sufficient for data and chunk size - see "Size of pool metadata LV" in the lvmthin(7) man page.

    2. Is it recommended to reduce/set metadata to a smaller value (using --poolmetadatasize).

      TL;DR: No.

      lvcreate selects appropriate size when creating the pool.

    3. Are there any performance impact / concerns by setting a lower metadata size.

      TL;DR: Yes.

      lvmthin man page at "Chunk size" says:

      When a thin pool is used primarily for the thin provisioning feature, a larger value is optimal. To optimize for many snapshots, a smaller value reduces copying time and consumes less space.

      If you create smaller metadata device, larger chunk size will be used. It is better to specify chunksize and let the tool calculate the size.

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