Cache block size warning

[Ciprian Hacman <ciprian.hacman@sematext.com>]

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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