Re: Extra RAM to improve OSD write performance ?

[Somnath Roy <Somnath.Roy@xxxxxxxxxxx>]

I doubt it will do much good in case of 100% write workload. You can tweak your VM dirty ration stuff to help the buffered write but the down side is the more amount of data it has to sync (while dumping dirty buffer eventually) the more spikiness it will induce..The write behavior won’t be smooth and gain won’t be much (or not at all).

But, Ceph does xattr reads in the write path, if you have very huge workload this extra RAM will help you to hold dentry caches in the memory (or go for sappiness setting not to swap out dentry caches) and effectively will save some disk hit. Also, in case of mixed read/write scenario this should help as some read could be benefitting from this. All depends on how random and how big is your workload..

 

 

Thanks & Regards

Somnath

 

From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Vickey Singh
Sent: Sunday, February 14, 2016 1:55 AM
To: ceph-users@xxxxxxxxxxxxxx; ceph-users
Subject: [ceph-users] Extra RAM to improve OSD write performance ?

 

Hello Community

 

Happy Valentines Day ;-)

 

I need some advice on using EXATA RAM on my OSD servers to improve Ceph's write performance.

 

I have 20 OSD servers each with 256GB RAM and 6TB x 16 OSD's, so assuming cluster is not recovering, most of the time system will have at least ~150GB RAM free. And for 20 machines its a lot ~3.0 TB RAM

 

Is there any way to use this free RAM to improve write performance of cluster. Something like Linux page cache for OSD write operations. 

 

I assume that by default Linux page cache can use free memory to improve OSD read performance ( please correct me if i am wrong). But how about OSD write improvement , How to improve that with free RAM.

 

PS : My Ceph cluster's workload is just OpenStack Cinder , Glance , Nova for instance disk

 

- Vickey - 

 

 

 

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