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A block needs to be written before read otherwise you will get funny result. For example, in case of flash (depending on how FW is implemented) , it will mostly
return you 0 if a block is not written. Now, I have seen some flash FW is really inefficient on manufacturing this data (say 0) if not written and some are really fast. So, to get predictable result you should be always reading a block that is written. In a device say half of the block is written and you are doing a full device
random reads , you will get unpredictable/spiky/imbalanced result. Same with rbd as well, consider it as a storage device and behavior would be similar. So, it is always recommended to precondition (fill up) a rbd image with
bigger block seq write before you do any synthetic test on that. Now, for filestore backend added advantage of preconditioning rbd will be the files in the filesystem will be created beforehand. Thanks & Regards Somnath From: V Plus [mailto:v.plussharp@xxxxxxxxx]
Thanks Somnath! As you recommended, I executed: dd if=/dev/zero bs=1M count=4096 of=/dev/rbd0 dd if=/dev/zero bs=1M count=4096 of=/dev/rbd1 Then the output results look more reasonable! Could you tell me why?? Btw, the purpose of my run is to test the performance of rbd in ceph. Does my case mean that before every test, I have to "initialize" all the images??? Great thanks!! On Sun, Dec 11, 2016 at 8:47 PM, Somnath Roy <Somnath.Roy@xxxxxxxxxxx> wrote: Fill up the image with big write (say 1M) first before reading and you should see sane throughput. Thanks & Regards Somnath From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx]
On Behalf Of V Plus Hi Guys, we have a ceph cluster with 6 machines (6 OSD per host). 1. I created 2 images in Ceph, and map them to another host A (outside the Ceph cluster). On host A, I got /dev/rbd0 and /dev/rbd1. 2. I start two fio job to perform READ test on rbd0 and rbd1. (fio job descriptions can be found below) "sudo fio fioA.job -output a.txt & sudo fio fioB.job -output b.txt & wait" 3. After the test, in a.txt, we got bw=1162.7MB/s, in b.txt, we get bw=3579.6MB/s. The results do NOT make sense because there is only one NIC on host A, and its limit is 10 Gbps (1.25GB/s). I suspect it is because of the cache setting. But I am sure that in file /etc/ceph/ceph.conf on host A,I already added: [client] rbd cache = false Could anyone give me a hint what is missing? why.... Thank you very much. fioA.job: [A] direct=1 group_reporting=1 unified_rw_reporting=1 size=100% time_based=1 filename=/dev/rbd0 rw=read bs=4MB numjobs=16 ramp_time=10 runtime=20 fioB.job: [B] direct=1 group_reporting=1 unified_rw_reporting=1 size=100% time_based=1 filename=/dev/rbd1 rw=read bs=4MB numjobs=16 ramp_time=10 runtime=20 Thanks... PLEASE NOTE: The information contained in this electronic mail message is intended only for the use of the designated recipient(s) named above. If the reader of this message is not the intended recipient, you are hereby notified that you
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