I just looked at pricing for the higher-end MLC devices, and it looks like I''m better off getting a single drive of 2X capacity than two with X capacity. Leaving aside the issue that by using 2 drives I get 2 x 3.0Gbps SATA performance instead of 1 x 3.0Gbps, are there problems with using two slices instead of whole-drives? That is, one slice for Read and the other for ZIL? My main concern is exactly how the on-drive cache would be used in a two-slices configuration. In order to get decent performance, I really need the on-drive cache to be used properly. -- Erik Trimble Java System Support Mailstop: usca22-123 Phone: x17195 Santa Clara, CA
Hi Erik, On 22/06/2009, at 1:15 PM, Erik Trimble wrote:> I just looked at pricing for the higher-end MLC devices, and it > looks like I''m better off getting a single drive of 2X capacity than > two with X capacity. > > Leaving aside the issue that by using 2 drives I get 2 x 3.0Gbps > SATA performance instead of 1 x 3.0Gbps, are there problems with > using two slices instead of whole-drives? That is, one slice for > Read and the other for ZIL?The benefit you will get using 2 drives instead of 1 will be doubling your IOPS which will improve your overall performance, especially when using those drives as ZILs. Are you planning on using these drives as primary data storage and ZIL for the same volumes or as primary storage for (say) your rpool and ZIL for a data pool on spinning metal? cheers, James
Erik Trimble wrote:> I just looked at pricing for the higher-end MLC devices, and it looks > like I''m better off getting a single drive of 2X capacity than two > with X capacity. > > Leaving aside the issue that by using 2 drives I get 2 x 3.0Gbps SATA > performance instead of 1 x 3.0Gbps, are there problems with using two > slices instead of whole-drives? That is, one slice for Read and the > other for ZIL? > > My main concern is exactly how the on-drive cache would be used in a > two-slices configuration. In order to get decent performance, I really > need the on-drive cache to be used properly.Is the on-disk cache volatile? For most SSDs I''m familiar with, the on-disk cache is non-volatile, so all of the rules pertaining to whole disks with volatile write buffers are nullified. -- richard
Richard Elling wrote:> Erik Trimble wrote: >> I just looked at pricing for the higher-end MLC devices, and it looks >> like I''m better off getting a single drive of 2X capacity than two >> with X capacity. >> >> Leaving aside the issue that by using 2 drives I get 2 x 3.0Gbps >> SATA performance instead of 1 x 3.0Gbps, are there problems with >> using two slices instead of whole-drives? That is, one slice for >> Read and the other for ZIL? >> >> My main concern is exactly how the on-drive cache would be used in a >> two-slices configuration. In order to get decent performance, I >> really need the on-drive cache to be used properly. > > Is the on-disk cache volatile? For most SSDs I''m familiar with, the > on-disk > cache is non-volatile, so all of the rules pertaining to whole disks with > volatile write buffers are nullified. > -- richard >I''m pretty sure it is volatile. It''s a single DRAM chip. http://benchmarkreviews.com/index.php?option=com_content&task=view&id=333&Itemid=60&limit=1&limitstart=3 (this review is specific to the OCZ Summit, but as the parts are pretty much Samsung-standard, it''s identical to several other brand''s versions) Are you sure most other SSDs have nvram as cache? I''m looking around, and they _seem_ to be using standard DRAM, just like Hard drives... Or maybe the SLC-based ones use nvram, and the MLC-based ones dram... -- Erik Trimble Java System Support Mailstop: usca22-123 Phone: x17195 Santa Clara, CA
James Lever wrote:> Hi Erik, > > On 22/06/2009, at 1:15 PM, Erik Trimble wrote: > >> I just looked at pricing for the higher-end MLC devices, and it looks >> like I''m better off getting a single drive of 2X capacity than two >> with X capacity. >> >> Leaving aside the issue that by using 2 drives I get 2 x 3.0Gbps >> SATA performance instead of 1 x 3.0Gbps, are there problems with >> using two slices instead of whole-drives? That is, one slice for >> Read and the other for ZIL? > > The benefit you will get using 2 drives instead of 1 will be doubling > your IOPS which will improve your overall performance, especially when > using those drives as ZILs. > > Are you planning on using these drives as primary data storage and ZIL > for the same volumes or as primary storage for (say) your rpool and > ZIL for a data pool on spinning metal? > > cheers, > James >ZIL and Read cache for a data pool of HDs. i.e. zpool create tank mirror c1t0d0 c1t1d0 [...] log c2t0d0 cache c3t0d0 or zpool create tank mirror c1t0d0 c1t1d0 [...] log c2t0d0s0 cache c2t0d0s1 -- Erik Trimble Java System Support Mailstop: usca22-123 Phone: x17195 Santa Clara, CA
comment below... Erik Trimble wrote:> Richard Elling wrote: >> Erik Trimble wrote: >>> I just looked at pricing for the higher-end MLC devices, and it >>> looks like I''m better off getting a single drive of 2X capacity than >>> two with X capacity. >>> >>> Leaving aside the issue that by using 2 drives I get 2 x 3.0Gbps >>> SATA performance instead of 1 x 3.0Gbps, are there problems with >>> using two slices instead of whole-drives? That is, one slice for >>> Read and the other for ZIL? >>> >>> My main concern is exactly how the on-drive cache would be used in a >>> two-slices configuration. In order to get decent performance, I >>> really need the on-drive cache to be used properly. >> >> Is the on-disk cache volatile? For most SSDs I''m familiar with, the >> on-disk >> cache is non-volatile, so all of the rules pertaining to whole disks >> with >> volatile write buffers are nullified. >> -- richard >> > I''m pretty sure it is volatile. It''s a single DRAM chip.Sure, but they put a supercap there which ensures that there is enough time to destage to flash before the supercap discharges -- nonvolatile. -- richard> > http://benchmarkreviews.com/index.php?option=com_content&task=view&id=333&Itemid=60&limit=1&limitstart=3 > > > (this review is specific to the OCZ Summit, but as the parts are > pretty much Samsung-standard, it''s identical to several other brand''s > versions) > > > Are you sure most other SSDs have nvram as cache? I''m looking around, > and they _seem_ to be using standard DRAM, just like Hard drives... > Or maybe the SLC-based ones use nvram, and the MLC-based ones dram... > >