[Gluster-users] Inviting comments on my plans

Fernando Frediani (Qube) fernando.frediani at qubenet.net
Mon Nov 19 17:12:43 UTC 2012

To grow the RAID is relatively simple and you can control how fast you want to rebuild to not impact your performance. You can even put some lines on your crontab to raise or decrease the priority of rebuild using the RAID controller CLI depending on the time. Yes RAID 5 you 'loose' the capacity of a disk, but you have to compromise somewhere. One disk isn't a big deal out of 8 or 12.
If you still decide to go with single disks and no RAID make sure that when a disk fails you won't get anything freezing or locked up until the disk is completely removed and declared dead by the kernel. I never did that myself to say how it will behaviour.

When you grow the RAID by adding more disks you grow the Logical Disk on the Raid controller first, then the partition(s) then the file system.
To be honest by the amount of data that you are talking about you will have I wouldn't even consider add a half populated server to grow later. As you are using small servers (12 slots) just add them fully populated and make the things easy not having to worry about grow RAID, but just replace disk when necessary.


-----Original Message-----
From: gluster-users-bounces at gluster.org [mailto:gluster-users-bounces at gluster.org] On Behalf Of Shawn Heisey
Sent: 19 November 2012 16:36
To: 'gluster-users at gluster.org'
Subject: Re: [Gluster-users] Inviting comments on my plans

On 11/19/2012 3:18 AM, Fernando Frediani (Qube) wrote:
> Hi,
> I agree with the comment about Fedora and wouldn't choose it a distribution, but if you are comfortable with it go ahead as I don't think this will be the major pain.
> RAID: I see where you are coming from to choose not have any RAID and I have thought myself before to do the same, mainly for performance reasons, but as mentioned how are you going to handle the drive swap ? If you think you can somehow automate it please share with us as I believe it is a major performance gain running the disks independently .

There will be no automation.  I'll have to do everything myself -- telling the RAID controller to make the disk available to the OS, putting a filesystem on it, re-adding it to gluster, etc.  Although drive failure is inevitable, I do not expect it to be a common occurrence.

> What you are willing to do with XFS+BRTFS I am not quiet sure it will work as you expect. Ideally you need to use snapshots from the Distributed Filesystem otherwise you might think you are getting a consistent copy of the data and you might not as you are not supposed to be reading/writing other than on the Gluster mount.

The filesystem will be mounted on /bricks/fsname, but gluster will be pointed at /bricks/fsname/volname.  I would put snapshots in /bricks/fsname/snapshots.  Gluster would never see the snapshot data.

> Performance: Simple and short - If you can compromise one disk per host AND choose to not go with independent disks(no RAID) go with RAID 5.
> As your system grows the reads and write should (in theory) be distributed across all bricks. If you have a disk failed you can easily replace it and even in a unlikely event that you lose two disks in a server and loose its data entirely you still have a copy of it in another place and can rebuild it with a bit of patience , so no data loss.
> Also we have had more than enough reports of bad performance in Gluster for all kinds of configurations (including RAID 10) so I don't think anyone should expect Gluster to perform that well, so using RAID 5, 6 or 10 underneath shouldn't make much difference and RAID 10 only would waste space. If you are storing bulk data (multimedia, images, big files) great, it will be streamed and sequential data and it should be ok and acceptable, but if you are storing things that do a lot of small IO or Virtual machines I'm not sure if Gluster is the best choice for you and you should think carefully about it.

A big problem that I would be facing if I went with RAID5 is that I won't initially have all drive bays populated.  The server has 12 drive bays.  If I populate 8 bays per server to start out, what happens when I need to fill in the other 4 bays?

If I make a new RAID5, then I have lost the capacity of another disk, and I have no option other than adding at least three drives at a time.  
I would not have the option of growing one disk at a time.  I can probably grow the existing RAID array, but that is a process that will literally take days, during which the entire array is in a fragile state with horrible performance. If others have experience with doing this on Dell hardware and have had consistently good luck with it, then my objection may be unfounded.

With individual disks instead of RAID, I can add one disk at a time to a server pair.

We will be storing photo, text, and video assets, currently about 80 million of them, with most of them being photos.  Each asset consists of a main file and a handful of very small metadata files.  If it's a video asset, then we actually have several "main" files - different formats and bitrates.  We have a website that acts as a front end to all this data.

Because of other systems (MySQL and Solr), we normally do not need to access the storage until someone wishes to see a detail page for an individual asset, or download the asset.  We have plans to migrate the primary metadata access to another system with better performance, possibly a NoSQL database.  We will keep the metadata files around so we have the ability to rebuild the primary system, but the goal is to only access the storage when we are retrieving the asset for a user download.  The systems that process incoming data would obviously need to access the storage often.


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