[ceph-users] Journal / WAL drive size?

David Byte dbyte at suse.com
Thu Nov 23 17:33:56 PST 2017


The answer tends to be “it depends”. For my test system with 144 6TB drives, I use a 50GB DB partition. In another case of testing, we have a ratio of about 10GB per TB.   What you have to watch out for is the performance difference when the DB overruns the SSD partition.

For the WAL, I provision 2GB and haven’t experienced any issues with that. You also probably will need to adjust the ratios, but that was covered in other threads previously.

David Byte
Sr. Technical Strategist
IHV Alliances and Embedded
SUSE

Sent from my iPhone. Typos are Apple's fault.

On Nov 23, 2017, at 3:19 PM, Rudi Ahlers <rudiahlers at gmail.com<mailto:rudiahlers at gmail.com>> wrote:

Hi Richard,

So do you rely on the CEPH to automatically decide the WAL device's location and size?

On Thu, Nov 23, 2017 at 4:04 PM, Richard Hesketh <richard.hesketh at rd.bbc.co.uk<mailto:richard.hesketh at rd.bbc.co.uk>> wrote:
Keep in mind that as yet we don't really have good estimates for how large bluestore metadata DBs may become, but it will be somewhat proportional to your number of objects. Considering the size of your OSDs, a 15GB block.db partition is almost certainly too small. Unless you've got a compelling reason not to you should probably partition your SSDs to use all the available space. Personally, I manually partition my block.db devices into even partitions and then invoke creation with

ceph-disk prepare --bluestore /dev/sdX --block.db /dev/disk/by-partuuid/whateveritis

Invoking by UUID is done because if presented an existing partition rather than the root block device it will just symlink precisely to the argument given, so using /dev/sdXYZ arguments is dangerous as they may not be consistent across hardware changes or even just reboots depending on your system.

Rich

On 23/11/17 09:53, Caspar Smit wrote:
> Rudi,
>
> First off all do not deploy an OSD specifying the same seperate device for DB and WAL:
>
> Please read the following why:
>
> http://docs.ceph.com/docs/master/rados/configuration/bluestore-config-ref/
>
>
> That said you have a fairly large amount of SSD size available so i recommend using it as block.db:
>
> You can specify a fixed size block.db size in ceph.conf using:
>
> [global]
> bluestore_block_db_size = 16106127360
>
> The above is a 15GB block.db size
>
> Now when you deploy an OSD with a seperate block.db device the partition will be 15GB.
>
> The default size is a percentage of the device i believe and not always a usable amount.
>
> Caspar
>
> Met vriendelijke groet,
>
> Caspar Smit
> Systemengineer
> SuperNAS
> Dorsvlegelstraat 13
> 1445 PA Purmerend
>
> t: (+31) 299 410 414<tel:%28%2B31%29%20299%20410%20414>
> e: casparsmit at supernas.eu<mailto:casparsmit at supernas.eu> <mailto:casparsmit at supernas.eu<mailto:casparsmit at supernas.eu>>
> w: www.supernas.eu<http://www.supernas.eu> <http://www.supernas.eu>
>
> 2017-11-23 10:27 GMT+01:00 Rudi Ahlers <rudiahlers at gmail.com<mailto:rudiahlers at gmail.com> <mailto:rudiahlers at gmail.com<mailto:rudiahlers at gmail.com>>>:
>
>     Hi,
>
>     Can someone please explain this to me in layman's terms. How big a WAL drive do I really need?
>
>     I have a 2x 400GB SSD drives used as WAL / DB drive and 4x 8TB HDD's used as OSD's. When I look at the drive partitions the DB / WAL partitions are only 576Mb & 1GB respectively. This feels a bit small.
>
>
>     root at virt1:~# lsblk
>     NAME               MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
>     sda                  8:0    0   7.3T  0 disk
>     ├─sda1               8:1    0   100M  0 part /var/lib/ceph/osd/ceph-0
>     └─sda2               8:2    0   7.3T  0 part
>     sdb                  8:16   0   7.3T  0 disk
>     ├─sdb1               8:17   0   100M  0 part /var/lib/ceph/osd/ceph-1
>     └─sdb2               8:18   0   7.3T  0 part
>     sdc                  8:32   0   7.3T  0 disk
>     ├─sdc1               8:33   0   100M  0 part /var/lib/ceph/osd/ceph-2
>     └─sdc2               8:34   0   7.3T  0 part
>     sdd                  8:48   0   7.3T  0 disk
>     ├─sdd1               8:49   0   100M  0 part /var/lib/ceph/osd/ceph-3
>     └─sdd2               8:50   0   7.3T  0 part
>     sde                  8:64   0 372.6G  0 disk
>     ├─sde1               8:65   0     1G  0 part
>     ├─sde2               8:66   0   576M  0 part
>     ├─sde3               8:67   0     1G  0 part
>     └─sde4               8:68   0   576M  0 part
>     sdf                  8:80   0 372.6G  0 disk
>     ├─sdf1               8:81   0     1G  0 part
>     ├─sdf2               8:82   0   576M  0 part
>     ├─sdf3               8:83   0     1G  0 part
>     └─sdf4               8:84   0   576M  0 part
>     sdg                  8:96   0   118G  0 disk
>     ├─sdg1               8:97   0     1M  0 part
>     ├─sdg2               8:98   0   256M  0 part /boot/efi
>     └─sdg3               8:99   0 117.8G  0 part
>       ├─pve-swap       253:0    0     8G  0 lvm  [SWAP]
>       ├─pve-root       253:1    0  29.3G  0 lvm  /
>       ├─pve-data_tmeta 253:2    0    68M  0 lvm
>       │ └─pve-data     253:4    0  65.9G  0 lvm
>       └─pve-data_tdata 253:3    0  65.9G  0 lvm
>         └─pve-data     253:4    0  65.9G  0 lvm
>
>
>
>
>     --
>     Kind Regards
>     Rudi Ahlers
>     Website: http://www.rudiahlers.co.za
>
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--
Richard Hesketh
Systems Engineer, Research Platforms
BBC Research & Development


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--
Kind Regards
Rudi Ahlers
Website: http://www.rudiahlers.co.za
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