For reads bigger block sizes are better (file size does not appear to matter). Depending on alignment a 8k file could cause 2 blocks to be read off of 2 separate disks, using more spindles and taking longer. The bigger the block io size on the raid disk the less likely with the same file size you have to read from 2 different disks. When I tested random read rates I found that no matter what the file size was a bigger block size on the raid was always better. It appear to come down to minimizing the number of disk spindles that have to be used for each file read. So my practice was always to set the block size to as big as possible, I believe some of the controllers I was using could go as high as 1mbyte. On Tue, Aug 19, 2014 at 1:38 PM, Chris Knipe <savage@xxxxxxxxxxxxx> wrote: > Hi All, > > I'm sitting with a bit of a catch 22 and need some feedback / inputs please. > This isn't strictly md related as all servers has MegaRAID SAS controllers > with BBUs and I am running hardware raid. So my apologies about the off > topic posting, but the theory remains the same I presume. All the servers > store millions of small (< 2mb) files, in a structured directory structure > to keep the amount of files per directory in check. > > Firstly, I have a bunch (3) of front end servers, all configured in RAID10 > and consisting of 8 x 4TB SATAIII drives. Up to now they have performed > very well, with roughly 30% reads and 70% writes. This is absolutely fine > as RAID10 does give much better write performance and we expect this. I > can't recall what the benches said when I tested this many, many months ago, > but it was good and IO wait even under heavy heavy usage is very little... > > The problem now is coming in that the servers are reaching their capacity > and the arrays are starting to fill up. Deleting files, isn't really an > option for me as I want to keep them as long as possible. So, let's get a > server to archive data on. > > So, a new server, 15 x 4TB SATAIII drives again, on a MegaRAID controller. > With the understanding that the "archives" will be read more than written to > (we only write here once we move data from the RAID10 arrays), I opted for > RAID5 rather. The higher spindle count surely should count for something. > Well. The server was configured, array initialised, and tests shows more > than 1gb/s in write speeds - faster than the RAID10 arrays. I am pleased! > > What's the problem? Well the front end servers does an enormous amount of > random read/writes (30/70 split), 24x7. Some 3 million files are added > (written) per day, of which roughly 30% are read again. So, the majority of > the IO activity is writing to disk. With all the writing going on, there is > effectively zero IO left for reading data. I can't read (or should we say > "move") data off the server faster than what it is being written. The > moment I start to do any amount of significant read requests, the IO wait > jumps through the roof and the write speeds obviously also crawl to a halt. > I suspect due to the seek time on the spindles, which does make sense and > all of that. So there still isn't really any problem here that we don't > know about already. > > Now, I realise that this is a really, really open question in terms of > interpretation, but what raid levels with high spindle counts (say 8, 12 or > 15 or so) will provide for the best "overall" and balanced read/write > performance in terms of random IO? I do not necessarily need blistering > performance in terms of speeds due to the small file sizes, but I do need > blistering fast performance in terms of IOPS and random read/writes... All > file systems currently EXT4 and all raid disks running with a 64K block > size. > > Many thanks, and once again my apologise for my theoretical question rather > than md specific question. > > -- > Chris. > > > > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
