Date: Tue, 26 Aug 2014 16:07:58 +0100 From: Andrew McRae <anm@pobox.com> To: Warren Block <wblock@wonkity.com> Cc: Michael Sierchio <kudzu@tenebras.com>, FreeBSD Questions <freebsd-questions@freebsd.org> Subject: Re: some ZFS questions Message-ID: <BCFC2D4F-68C2-41CE-9459-062FB641EE3C@pobox.com> In-Reply-To: <alpine.BSF.2.11.1408242118330.2348@wonkity.com> References: <201408070816.s778G9ug015988@sdf.org> <40AF5B49-80AF-4FE2-BA14-BFF86164EAA8@kraus-haus.org> <201408211007.s7LA7YGd002430@sdf.org> <E32981BF-613C-49D9-B7F8-18B9A99C0141@kraus-haus.org> <201408241015.s7OAFRv7010196@sdf.org> <20140825014249.GA28995@neutralgood.org> <CAHu1Y71U1CnU=iCdNf902YiLzaJ4TtpZMwpjN6xms_Xuwsbu9Q@mail.gmail.com> <alpine.BSF.2.11.1408242059290.2348@wonkity.com> <CAHu1Y70_EM3JUS_dfKv2gm8GCfM7KEbP4q7JmYSG5o%2BD=vFjyA@mail.gmail.com> <alpine.BSF.2.11.1408242118330.2348@wonkity.com>
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On 25 Aug 2014, at 04:23, Warren Block <wblock@wonkity.com> wrote: > On Sun, 24 Aug 2014, Michael Sierchio wrote: >=20 >> On Sun, Aug 24, 2014 at 8:00 PM, Warren Block <wblock@wonkity.com> = wrote: >>> On Sun, 24 Aug 2014, Michael Sierchio wrote: >>>=20 >>>> On Sun, Aug 24, 2014 at 6:42 PM, <kpneal@pobox.com> wrote: >>>>=20 >>>>=20 >>>>> If you can use GPT partitioning then you can have GPT labels on >>>>> partitions >>>>> spanning entire disks. Then you can rearrange physical disks to = your >>>>> hearts >>>>> content. >>>>>=20 >>>>> The bonus is that you can put identifying information in the label = name >>>>> to be even more sure you are replacing the correct disk when the = time >>>>> comes. >>>>=20 >>>>=20 >>>> You can do this - and it's the approach that FreeNAS takes, but you >>>> lose the benefits of presenting a whole disk to ZFS - the ability = to >>>> fully use the disk cache when presented a raw device rather than a >>>> partition. >>>=20 >>>=20 >>> I thought that was a Solaris-only limitation. In fact, I'm pretty = sure it >>> is, and does not apply on FreeBSD. >>=20 >> You mean Solaris-only benefit? >=20 > No, I mean that the disk cache problem was only on Solaris, and does = not apply to FreeBSD. On FreeBSD, there is no difference in performance = between partitions versus the whole disk. Actually the problem never existed in the first place. It=92s an urban = myth based on a misunderstanding of the fact that when presented with a = whole disk, ZFS tries to enable the disk=92s write cache. (And that = generally has no effect, except for some old Sun-branded disks that = shipped with the write cache disabled, as I understand it.) Here=92s the code, still in FreeBSD: = https://svnweb.freebsd.org/base/stable/10/sys/cddl/contrib/opensolaris/uts= /common/fs/zfs/vdev_disk.c?view=3Dmarkup#l569 Another comment: http://marc.info/?l=3Dzfs-discuss&m=3D134022989322125
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