[linux-lvm] Cache block size warning

[Ciprian Hacman]

Hi,

We are configuring lvm with SSD caching on some of our boxes in AWS. When
the cache is initialised, we see this in logs:

device-mapper*: cache: You have created a cache device with a lot of
individual cache blocks (4915200)*

*               All these mappings can consume a lot of kernel memory, and
take some time to read/write.*

*               Please consider increasing the cache block size to reduce
the overall cache block count.*


Is this something to worry? If so, how can we improve it?

For reference, the output of lsblk is:

NAME                    MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT

xvda                    202:0    0    25G  0 disk

└─xvda1                 202:1    0    25G  0 part  /

xvdb                    202:16   0 152.6G  0 disk

└─md0                     9:0    0 304.9G  0 raid0

  ├─storage-cache_cdata 252:0    0   300G  0 lvm

  │ └─storage-data      252:3    0   1.5T  0 lvm   /mnt/local

  └─storage-cache_cmeta 252:1    0     1G  0 lvm

    └─storage-data      252:3    0   1.5T  0 lvm   /mnt/local

xvdc                    202:32   0 152.6G  0 disk

└─md0                     9:0    0 304.9G  0 raid0

  ├─storage-cache_cdata 252:0    0   300G  0 lvm

  │ └─storage-data      252:3    0   1.5T  0 lvm   /mnt/local

  └─storage-cache_cmeta 252:1    0     1G  0 lvm

    └─storage-data      252:3    0   1.5T  0 lvm   /mnt/local

xvdg                    202:96   0   1.5T  0 disk

└─storage-data_corig    252:2    0   1.5T  0 lvm

  └─storage-data        252:3    0   1.5T  0 lvm   /mnt/local


Thanks,
Ciprian
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