[ovirt-users] strange iscsi issue

[Yaniv Kaul]

On 08/09/15 09:05, Alex McWhirter wrote:
> Are we talking about a single ssd or an array of them? VMs are usually large continuous image files. SSDs are faster delivering many small files over large continuous file.
>
> I believe ovirt forces sync writes by default, but I'm not sure as I'm using NFS. The best thing to do is figure out whether it's a storage issue or network issue.
>
> Try setting your iscsi server to use async writes, this can be dangerous if either server crashes or loses power so I would just do it for testing purposes. 

I do not recommend it, as it would not reflect real life usage later on.

>
> With async writes you should be able to hit near 10gbps writes, but reads will depend on how much data is cached and how much ram the iscsi server has.
>
> Are you presenting a raw disk over iscsi, an image file, or a filesystem lun via zfs or something similar?

Some tips when using iSCSI and general IO performance:
- The VM should be using VirtIO for both disk and NIC.
- I recommend XFS over EXT4, but both are OK generally. If possible,
however, I'd test with a raw block device first.
- Ensure you have enough paths to the storage and/or multiple iSCSI
sessions. You may wish to configure the iSCSI target portal with
multiple IP addresses (not only for redundancy, but for multiple
connections).
- I highly recommend 'fio' as an IO tool over Bonnie or hdparm.   


Is the VM CPU the bottleneck, perhaps?
Y.

>
> Alex sent the message, but his phone sent the typos...
>
> On Sep 8, 2015 1:45 AM, Karli Sjöberg wrote: > > tis 2015-09-08 klockan 06:59 +0200 skrev Demeter Tibor: > > Hi, > > Thank you for your reply. > > I'm sorry but I don't think so. This storage is fast, because it is a SSD based storage, and I can read/write to it with fast performance. > > I know, in virtual environment the I/O always slowest than on physical, but here I have a very large difference. > > Also, I use ext4 FS. > > My suggestion would be to use a filesystem benchmarking tool like bonnie > ++ to first test the performance locally on the storage server and then > redo the same test inside of a virtual machine. Also make sure the VM is > using VirtIO disk (either block or SCSI) for best performance. I have > tested speeds over 1Gb/s with bonded 1Gb NICS so I know it should work > in theory as well as practice. > > Oh, and for the record. IO doesn´t have to be bound by the speed of > storage, if the host caches in RAM before sending it over the wire. But > that in my opinion is dangerous and as far as I know, it´s not actived > in oVirt, please correct me if I´m wrong. > > /K > > > > > Thanks > > > > Tibor > > > > > > ----- 2015. szept.. 8., 0:40, Alex McWhirter alexmcwhirter at triadic.us írta: > > > > > Unless you're using a caching filesystem like zfs, then you're going to be > > > limited by how fast your storage back end can actually right to disk. Unless > > > you have a quite large storage back end, 10gbe is probably faster than your > > > disks can read and write. > > > > > > On Sep 7, 2015 4:26 PM, Demeter Tibor wrote: > > >> > > >> Hi All, > > >> > > >> I have to create a test environment for testing purposes, because we need to > > >> testing our new 10gbe infrastructure. > > >> One server that have a 10gbe nic - this is the vdsm host and ovirt portal. > > >> One server that have a 10gbe nic - this is the storage. > > >> > > >> Its connected to each other throught a dlink 10gbe switch. > > >> > > >> Everything good and nice, the server can connect to storage, I can make and run > > >> VMs, but the storage performance from inside VM seems to be 1Gb/sec only. > > >> I did try the iperf command for testing connections beetwen servers, and it was > > >> 9.40 GB/sec. I have try to use hdparm -tT /dev/mapper/iscsidevice and also it > > >> was 400-450 MB/sec. I've got same result on storage server. > > >> > > >> So: > > >> > > >> - hdparm test on local storage ~ 400 mb/sec > > >> - hdparm test on ovirt node server through attached iscsi device ~ 400 Mb/sec > > >> - hdparm test from inside vm on local virtual disk - 93-102 Mb /sec > > >> > > >> The question is : Why? > > >> > > >> ps. I Have only one ovirtmgmt device, so there are no other networks. The router > > >> is only 1gbe/sec, but i've tested and the traffic does not going through  this. > > >> > > >> Thanks in advance, > > >> > > >> Regards, > > > > Tibor > > _______________________________________________ > > Users mailing list > > Users at ovirt.org > > http://lists.ovirt.org/mailman/listinfo/users >
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