[linux-lvm] Cache block size warning

[linux-lvm] Cache block size warning

[Ciprian Hacman]

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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
--
Performance Monitoring * Log Analytics * Search Analytics
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Re: [linux-lvm] Cache block size warning

[Zdenek Kabelac]

Dne 19.11.2015 v 08:27 Ciprian Hacman napsal(a):
> 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?
>

If you have lots of kernel memory - it's probably ok.
Otherwise you may try to use bigger chunk size.

e.g.  --chunksize 512K   or even more:

lvcreate --type cache-pool -L300G vg/cpool

lvcreate -H --chunksize 512K -L1.5T   --name data  vg/cpool

It's possible future version of lvm2 will try to create this by default.

Zdenek

Re: [linux-lvm] Cache block size warning

[Ciprian Hacman]

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Thank you for the quick answer. Sounds easy to implement if needed.

Ciprian
--
Performance Monitoring * Log Analytics * Search Analytics
Solr & Elasticsearch Support * http://sematext.com/

On Thu, Nov 19, 2015 at 10:56 AM, Zdenek Kabelac <zdenek.kabelac@gmail.com>
wrote:

> Dne 19.11.2015 v 08:27 Ciprian Hacman napsal(a):
>
>> 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?
>>
>>
> If you have lots of kernel memory - it's probably ok.
> Otherwise you may try to use bigger chunk size.
>
> e.g.  --chunksize 512K   or even more:
>
> lvcreate --type cache-pool -L300G vg/cpool
>
> lvcreate -H --chunksize 512K -L1.5T   --name data  vg/cpool
>
> It's possible future version of lvm2 will try to create this by default.
>
> Zdenek
>
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>

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