Date: Mon, 4 Jul 2016 08:05:40 +0200 From: Ben RUBSON <ben.rubson@gmail.com> To: freebsd-fs@freebsd.org Subject: Re: HAST + ZFS + NFS + CARP Message-ID: <B48FB28E-30FA-477F-810E-DF4F575F5063@gmail.com> In-Reply-To: <65906F84-CFFC-40E9-8236-56AFB6BE2DE1@ixsystems.com> References: <20160630144546.GB99997@mordor.lan> <71b8da1e-acb2-9d4e-5d11-20695aa5274a@internetx.com> <AD42D8FD-D07B-454E-B79D-028C1EC57381@gmail.com> <20160630153747.GB5695@mordor.lan> <63C07474-BDD5-42AA-BF4A-85A0E04D3CC2@gmail.com> <678321AB-A9F7-4890-A8C7-E20DFDC69137@gmail.com> <20160630185701.GD5695@mordor.lan> <6035AB85-8E62-4F0A-9FA8-125B31A7A387@gmail.com> <20160703192945.GE41276@mordor.lan> <20160703214723.GF41276@mordor.lan> <65906F84-CFFC-40E9-8236-56AFB6BE2DE1@ixsystems.com>
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> On 04 Jul 2016, at 00:24, Jordan Hubbard <jkh@ixsystems.com> wrote: >=20 >> On Jul 3, 2016, at 2:47 PM, Julien Cigar <julien@perdition.city> = wrote: >>=20 >> I guess that ZFS will split the read requests accross all devices in >> order to maximize performance... which could lead to contrary to what = is >> expecpted when iSCSI disks are involved, no? >> Is there some sysctl params which could prevent this unexpected >> behavior? >=20 > Nope. You will suffer the performance implications of layering a = filesystem that expects =E2=80=9Crotating media or SSDs=E2=80=9D (with = the innate ability to parallelize multiple requests in a way that ADD = performance) on top of a system which is now serializing the requests = across an internet connection to another software layer which may offer = no performance benefits to having multiple LUNs at all. You can try = iSCSI-specific tricks like MPIO to try and increase performance, but ZFS = itself is just going to treat everything it sees as =E2=80=9Ca disk=E2=80=9D= and so physical concepts like mirrors or multiple vdevs for performance = won=E2=80=99t translate across. >=20 > Example question: What=E2=80=99s the point of writing multiple copies = of data across virtual disks in a mirror configuration if the underlying = storage for the virtual disks is already redundant and the I/Os to it = serialize? > Example Answer: There is no point. In fact, it=E2=80=99s a = pessimization to do so. Of course Jordan, in this topic, we (well at least me :) make the = following assumption : one iSCSI target/disk =3D one real physical disk (a SAS disk, a SSD = disk...), from a server having its own JBOD, no RAID adapter or = whatever, just what ZFS likes ! > This is not a lot different than running ZFS on top of RAID = controllers that turn N physical disks into 1 or more virtual disks. = You have to make entirely different performance decisions based on such = scenarios and that=E2=80=99s just the way it is, which is also why we = don=E2=80=99t recommend doing that. Of course you loose all ZFS benefits if you only mirror 2 "disks", a big = one from storage array A, the same from storage array B. No interest.
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