Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
Andrew Gabriel andrew.gabr...@oracle.com wrote: If you go back to the late 1970's before tracks had embedded servo data, on multi-platter disks you had one surface which contained the head positioning servo data, and the drive relied on accurate vertical alignment between heads/surfaces to keep on track (and drives could head-switch instantly). Around 1980, tracks got too close together for this to work anymore, and the servo positioning data was embedded into each track itself. The very first drives of this type scanned all the The first drive I am aware to use embedded servo was the Simemens MegaFile drive series in 1986 and while it could increase the data density, it caused a slow down for the head switch time. I was forced to write my own disk formatting program in order to be able to apply a track skew value != 0 to compensate this problem. Fortunately, I did this together with introducing a SCSI generic driver so I was able to format disks from a running OS and was not forced to boot the Sun standalone disk formatting program anymore ;-) Jörg -- EMail:jo...@schily.isdn.cs.tu-berlin.de (home) Jörg Schilling D-13353 Berlin j...@cs.tu-berlin.de(uni) joerg.schill...@fokus.fraunhofer.de (work) Blog: http://schily.blogspot.com/ URL: http://cdrecord.berlios.de/private/ ftp://ftp.berlios.de/pub/schily ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
On Sat, Feb 5, 2011 at 3:34 PM, Roy Sigurd Karlsbakk r...@karlsbakk.net wrote: so as not to exceed the channel bandwidth. When they need to get higher disk capacity, they add more platters. May this mean those drives are more robust in terms of reliability, since the leaks between sectors is less likely with the lower density? More platters leads to more heat and higher power consumption. Most drives are 3 or 4 platters, though Hitachi usually manufactures 5 platter drives as well. -B -- Brandon High : bh...@freaks.com ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
One characteristic people often overlook is: When you get a disk with higher capacity (say, 2T versus 600G) then you get more empty space and hence typically lower fragmentation in the drive. Also, the platter density is typically higher, so if the two drives have equal RPM's, typically the higher capacity drive can perform faster sustained sequential operations. 10k and 15k drives aren't true 3,5, but closer to 2,5, even though the casing is the standard 3,5 size (open one if you doubt this). Usually, these drives have similar density as their respective 7k2 drives, and thus higher speed because of the spin rate. Vennlige hilsener / Best regards roy -- Roy Sigurd Karlsbakk (+47) 97542685 r...@karlsbakk.net http://blogg.karlsbakk.net/ -- I all pedagogikk er det essensielt at pensum presenteres intelligibelt. Det er et elementært imperativ for alle pedagoger å unngå eksessiv anvendelse av idiomer med fremmed opprinnelse. I de fleste tilfeller eksisterer adekvate og relevante synonymer på norsk. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
Nope. Most HDDs today have a single read channel, and they select which head uses that channel at any point in time. They cannot use multiple heads at the same time, because the heads to not travel the same path on their respective surfaces at the same time. There's no real vertical alignment of the tracks between surfaces, and every surface has its own embedded position information that is used when that surface's head is active. There were attempts at multi-actuator designs with separate servo arms and multiple channels, but mechanically they're too difficult to manufacture at high yields as I understood it. Perhaps a stupid question, but why don't they read from all platters in parallel? Vennlige hilsener / Best regards roy -- Roy Sigurd Karlsbakk (+47) 97542685 r...@karlsbakk.net http://blogg.karlsbakk.net/ -- I all pedagogikk er det essensielt at pensum presenteres intelligibelt. Det er et elementært imperativ for alle pedagoger å unngå eksessiv anvendelse av idiomer med fremmed opprinnelse. I de fleste tilfeller eksisterer adekvate og relevante synonymer på norsk. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
Roy Sigurd Karlsbakk wrote: Nope. Most HDDs today have a single read channel, and they select which head uses that channel at any point in time. They cannot use multiple heads at the same time, because the heads to not travel the same path on their respective surfaces at the same time. There's no real vertical alignment of the tracks between surfaces, and every surface has its own embedded position information that is used when that surface's head is active. There were attempts at multi-actuator designs with separate servo arms and multiple channels, but mechanically they're too difficult to manufacture at high yields as I understood it. Perhaps a stupid question, but why don't they read from all platters in parallel? The answer is in the text you quoted above. There are drives now with two level actuators. The primary actuator is the standard actuator you are familiar with which moves all the arms. The secondary actuator is a piezo crystal towards the head end of the arm which can move the head a few tracks very quickly without having to move the arm, and these are one per head. In theory, this might allow multiple heads to lock on to their respective tracks at the same time for parallel reads, but I haven't heard that they are used in this way. If you go back to the late 1970's before tracks had embedded servo data, on multi-platter disks you had one surface which contained the head positioning servo data, and the drive relied on accurate vertical alignment between heads/surfaces to keep on track (and drives could head-switch instantly). Around 1980, tracks got too close together for this to work anymore, and the servo positioning data was embedded into each track itself. The very first drives of this type scanned all the surfaces on startup to build up an internal table of the relative misalignment of tracks across the surfaces, but this rapidly became unviable as drive capacity increased and this scan would take an unreasonable length of time. It may be that modern drives learn this as they go - I don't know. -- Andrew Gabriel ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
For the data sheet I referenced, all the drive sizes have the same sustained data rate OD, 125 MB/s. Eric posted an explanation for this, which seems entirely believable: The data rate is not being limited by the density of magnetic material on the platter or the rotational speed, but by the head or channel bandwidth to each platter itself. When they run the disks at a higher RPM, they need to stretch the bits longer on the disk surface so as not to exceed the channel bandwidth. When they need to get higher disk capacity, they add more platters. May this mean those drives are more robust in terms of reliability, since the leaks between sectors is less likely with the lower density? Vennlige hilsener / Best regards roy -- Roy Sigurd Karlsbakk (+47) 97542685 r...@karlsbakk.net http://blogg.karlsbakk.net/ -- I all pedagogikk er det essensielt at pensum presenteres intelligibelt. Det er et elementært imperativ for alle pedagoger å unngå eksessiv anvendelse av idiomer med fremmed opprinnelse. I de fleste tilfeller eksisterer adekvate og relevante synonymer på norsk. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- boun...@opensolaris.org] On Behalf Of taemun Uhm. Higher RPM = higher linear speed of the head above the platter = higher throughput. If the bit pitch (ie the size of each bit on the platter) is the Nope. That's what I originally said, but I was proven wrong. For the data sheet I referenced, all the drive sizes have the same sustained data rate OD, 125 MB/s. Eric posted an explanation for this, which seems entirely believable: The data rate is not being limited by the density of magnetic material on the platter or the rotational speed, but by the head or channel bandwidth to each platter itself. When they run the disks at a higher RPM, they need to stretch the bits longer on the disk surface so as not to exceed the channel bandwidth. When they need to get higher disk capacity, they add more platters. This would logically conclude that you can get a higher maximum disk capacity at a rotational speed which is smaller. In fact, I currently see up to 3T available in 7.2krpm drives ... I see a max 800G in 15krpm... Yes, the higher rpm drives have smaller latency. No, they don't have higher sustained throughput. If anyone wants to look up more drive specs... Here's how to find it on seagate.com: Go to support, Knowledgebase. Under Support go to Document Library. Click the drive in question. And then you can find the Data Sheet. The couple of things that are really clear by looking over a bunch of data sheets are: * Higher rpm's means lower latency. (duh.) * Higher rpm's is loosely correlated with higher throughput, but it's not a linear correlation, and not always present. * If you go to a different drive type (SATA vs SAS vs FC) then you can get higher throughput... In no case is the throughput even remotely close to the bus speed, so the improved performance is not *because* of the interface. Presumably the more expensive drive type has a more expensive head or whatever internally. * Larger disk size does not improve sustained throughput at all. Zero. All of this supports what Eric said. The throughput of a drive is not determined by the platter density or rotation speed. It's limited by the head or something else in the data channel accessing the disk. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
Edward, Thanks for the reply. Good point on platter density. I'ld considered the benefit of lower fragmentation but not the possible increase in sequential iops due to density. I assume while a 2TB 7200rpm drive may have better sequential IOPS than a 500GB, it will not be double and therefore, if the 500GB's are half price the double spindle count would lead to better overall sequential IOPS (assuming still enough excess space to remove fragmentation benefit and increased usable GB's). Agree? Our IO is random small(4kB) read/write but I expect ZFS to convert the writes to sequential and the L2ARC to intercept a lot of the reads. (assumes low latency, high iop ZIL SLOG device). It basically seems to come down to whether the random reads that miss the cache need lower individual latency or not. Thanks for the pointer on terminology. I had thought that the RAM write cache was also called ARC (in addition to the RAM read cache). -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- boun...@opensolaris.org] On Behalf Of James I assume while a 2TB 7200rpm drive may have better sequential IOPS than a 500GB, it will not be double and therefore, Don't know why you'd assume that. I would assume a 2TB drive would be precisely double the sequential throughput of a 500G. I think if you double the surface density in two dimensions (a flat surface) you end up with 4x the storage capacity. Hence, a 2T drive should have 2x the 1-dimensional track density, and should be 2x faster sequential throughput than a 500G drive, with all other things being equal. Our IO is random small(4kB) read/write but I expect ZFS to convert the writes to sequential and the L2ARC to intercept a lot of the reads. (assumes low latency, high iop ZIL SLOG device). If you truly have random reads, then your L2ARC can't help you much. Your L2ARC can only help if you have a lot of repeated reads clustered in hot areas of the storage pool. ARC is based on Frequently Used and Recently Used ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
On Feb 2, 2011, at 6:10 AM, Edward Ned Harvey wrote: From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- boun...@opensolaris.org] On Behalf Of James I assume while a 2TB 7200rpm drive may have better sequential IOPS than a 500GB, it will not be double and therefore, Don't know why you'd assume that. I would assume a 2TB drive would be precisely double the sequential throughput of a 500G. I think if you double the surface density in two dimensions (a flat surface) you end up with 4x the storage capacity. Hence, a 2T drive should have 2x the 1-dimensional track density, and should be 2x faster sequential throughput than a 500G drive, with all other things being equal. They aren't. Check the datasheets, the max media bandwidth is almost always published. -- richard ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
From: Richard Elling [mailto:richard.ell...@gmail.com] They aren't. Check the datasheets, the max media bandwidth is almost always published. I looked for said data sheets before posting. Care to drop any pointers? I didn't see any drives publishing figures for throughput to/from platter today. I know this information exists for some drives. I've seen it before. But it's apparently not for most of the readily available drives on the market now. Not in the dozen drives that I tried to look up. The goal is to compare the throughput for two drives, where size of one drive is 4x greater than the size of the other, and all other things are equal. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
On Wed, Feb 2, 2011 at 6:10 AM, Edward Ned Harvey opensolarisisdeadlongliveopensola...@nedharvey.com wrote: Don't know why you'd assume that. I would assume a 2TB drive would be precisely double the sequential throughput of a 500G. I think if you double That's assuming that the drives have the same number of platters. 500G drives are generally one platter, and 2T drives are generally 4 platters. Same size platters, same density. The 500G drive could be expected to have slightly higher random iops due to lower mass in the heads, but it's probably not statistically significant. I think the current batch of 3TB drives are 7200 RPM with 5 platters and 667GB per platter or 5400 RPM with 4 platters at 750GB/platter. -B -- Brandon High : bh...@freaks.com ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
Thanks Richard Edward for the additional contributions. I had assumed that maximum sequential transfer rates on datasheets (btw - those are the same for differing capacity seagate's) were based on large block sizes and a ZFS 4kB recordsize* would mean much lower IOPS. e.g. Seagate Constellations are around 75-141MB/s(inner-outer) and 75MB/s is 18750 4kB IOPS! However I've just tested** a slow 1TB 7200 drive and got over 6000 4kB seq write IOPS which is a lot more than the 500 I was working on. If this is correct then even with hardly any spindles, if there's enough free space for ZFS to do sequential writes (no interrupting reads etc), you should easily get 6000 4k write IOPS from the disks (ie SLOG IOPS/latency will become limiting factor for sync writes) However, is this what people are actually seeing?(links to any other good reference builds with benchmarks welcome). The benchmarks I've found so far are: 1) http://www.zfsbuild.com/2010/10/09/nexenta-core-platform-benchmarks/ (maxed around 4000 write IOPS but iSCSI(sync) to X25-E so could be limited by that - rated 3300 IOPS) 2) http://www.opensolaris.org/jive/thread.jspa?messageID=507090#507090 One response quoted achieving 550-600 write IOPS on 15k drives (actually just realised this is your response Edward) (if your traffic is large blocks this may explain lower iops if bandwidth limited?). ps. Regarding workload and Random reads/L2ARC: The VMs are Windows Servers (Web servers, Databases etc) so the overall mix is random but I'm expecting L2ARC should end up holding frequently read blocks like those behind regularly read database blocks and, especially if we got dedupe working, the Operating System blocks. I also theorise that with thick vmdk files's and defrag'd guest OS and applications, ZFS read-ahead should start to kick in when sequential reads are made to files within the OS. * VMWare over NFS scenario (similar to local database - 4/8kB read/writes to large file) ** IOMeter, 4kB 100% seq, 1 64 outstanding, Win7 partition 4kb alloc unit, Drive write cache enabled (I believe ZFS uses drive write cache) on WD10EACS ). . -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
From: Brandon High [mailto:bh...@freaks.com] That's assuming that the drives have the same number of platters. 500G drives are generally one platter, and 2T drives are generally 4 platters. Same size platters, same density. The 500G drive could be Wouldn't multiple platters of the same density still produce a throughput that's a multiple of what it would have been with a single platter? I'm assuming the heads on the multiple platters are all able to operate simultaneously. Anyway, here's a data point: http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_7200_12.pdf All the disks from 160G up to 1T have the same sustained data rate, which is 125 MB/s ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- boun...@opensolaris.org] On Behalf Of James block sizes and a ZFS 4kB recordsize* would mean much lower IOPS. e.g. Seagate Constellations are around 75-141MB/s(inner-outer) and 75MB/s is 18750 4kB IOPS! However I've just tested** a slow 1TB 7200 drive and got over 6000 4kB seq write IOPS which is a lot more than the 500 I was working For sustained throughput, I don't measure in IOPS. I measure in MB/s, or Mbit/s. For a slow hard drive, 500Mbit/s. For a fast one, 1 Gbit/s or higher. I was surprised by the specs of the seagate disks I just emailed a moment ago. 1Gbit out of a 7.2krpm drive... That's what I normally expect out of a 15krpm drive. I know people sometimes (often) use IOPS even when talking about sequential operations, but I only say IOPS for random operations. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
On Wed, Feb 2 at 20:40, Edward Ned Harvey wrote: Wouldn't multiple platters of the same density still produce a throughput that's a multiple of what it would have been with a single platter? I'm assuming the heads on the multiple platters are all able to operate simultaneously. Nope. Most HDDs today have a single read channel, and they select which head uses that channel at any point in time. They cannot use multiple heads at the same time, because the heads to not travel the same path on their respective surfaces at the same time. There's no real vertical alignment of the tracks between surfaces, and every surface has its own embedded position information that is used when that surface's head is active. There were attempts at multi-actuator designs with separate servo arms and multiple channels, but mechanically they're too difficult to manufacture at high yields as I understood it. http://www.tomshardware.com/news/seagate-hdd-harddrive,8279.html -- Eric D. Mudama edmud...@bounceswoosh.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
On Wed, Feb 2 at 20:45, Edward Ned Harvey wrote: For sustained throughput, I don't measure in IOPS. I measure in MB/s, or Mbit/s. For a slow hard drive, 500Mbit/s. For a fast one, 1 Gbit/s or higher. I was surprised by the specs of the seagate disks I just emailed a moment ago. 1Gbit out of a 7.2krpm drive... That's what I normally expect out of a 15krpm drive. It used to be that enterprise grade, higher RPM devices used more expensive electronics, but that's not really the case anymore. It seems most vendors are trying to use common electronics across their product lines, which generally makes great business sense. These days I think most HDD companies get their channel working at a certain max bitrate, and format their drive zones to match that bitrate at the max radius where velocity is the highest. This is a bit of a simplification, but it's the general idea. When the drive is spinning the media less quickly, in a 7200 RPM device, they can pack the bits in more tightly, which lowers overall cost because they need fewer heads and platters to achieve a target capacity. It just so happens that the max bits/second flying under the read head is a constant pegged to the channel design. All other things being equal, the 15k and the 7200 drive, which share electronics, will have the same max transfer rate at the OD. I know people sometimes (often) use IOPS even when talking about sequential operations, but I only say IOPS for random operations. Me too, though not everyone realizes how much overhead there can be in small operations, even sequential ones. --eric -- Eric D. Mudama edmud...@bounceswoosh.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
On Feb 2, 2011, at 8:10 PM, Eric D. Mudama wrote: All other things being equal, the 15k and the 7200 drive, which share electronics, will have the same max transfer rate at the OD. Is that true? So the only difference is in the access time? Mark ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
Uhm. Higher RPM = higher linear speed of the head above the platter = higher throughput. If the bit pitch (ie the size of each bit on the platter) is the same, then surely a higher linear speed corresponds with a larger number of bits per second? So if all other things being equal includes the bit density, and radius to the edge of the media, then ... surely higher rpm = higher throughput? Cheers, On 3 February 2011 14:10, Mark Sandrock mark.sandr...@oracle.com wrote: On Feb 2, 2011, at 8:10 PM, Eric D. Mudama wrote: All other things being equal, the 15k and the 7200 drive, which share electronics, will have the same max transfer rate at the OD. Is that true? So the only difference is in the access time? Mark ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
On Thu, Feb 3 at 14:18, taemun wrote: Uhm. Higher RPM = higher linear speed of the head above the platter = higher throughput. If the bit pitch (ie the size of each bit on the platter) is the same, then surely a higher linear speed corresponds with a larger number of bits per second? So if all other things being equal includes the bit density, and radius to the edge of the media, then ... surely higher rpm = higher throughput? Cheers, Point being that they have to lower the bit density on high RPM drives to fit within the bandwidth constraints of the channel. If they could just get their channel working at 3GHz instead of 2GHz or whatever, they'd use that capability to pack even more bits into the consumer drives to lower costs. -- Eric D. Mudama edmud...@bounceswoosh.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- boun...@opensolaris.org] On Behalf Of James I’m trying to select the appropriate disk spindle speed for a proposal and would welcome any experience and opinions (e.g. has anyone actively chosen 10k/15k drives for a new ZFS build and, if so, why?). There is nothing special about ZFS in relation to spindle speed. If you get higher rpm's, then you get higher iops, and the same is true for EXT3, NTFS, HFS+, ZFS, etc. One characteristic people often overlook is: When you get a disk with higher capacity (say, 2T versus 600G) then you get more empty space and hence typically lower fragmentation in the drive. Also, the platter density is typically higher, so if the two drives have equal RPM's, typically the higher capacity drive can perform faster sustained sequential operations. Even if you use slow drives, assuming you have them in some sort of raid configuration, they quickly add up sequential speed to reach the bus speed. So if you expect to do large sequential operations, go for the lower rpm disks. But if you expect to do lots of small operations, then twice the rpm's literally means twice the performance. So for small random operations, go for the higher rpm disks. ** My understanding is that ZFS will adjust the amount of data accepted into each “transaction” (TXG) to ensure it can be written to disk in 5s.Async data will stay in ARC, Sync data will also go to ZIL or, if overthreshold, will go to disk and pointer to ZIL(on low latency SLOG) – ie. all writes apart from sync writes ZFS will aggregate small random writes into larger sequential writes. So you don't have to worry too much about rpm's and iops during writes. But of course there's nothing you can do about the random reads. So if you do random reads, you do indeed want higher rpm's. Your understanding (or terminology) of arc is not correct. Arc and l2arc are read cache. The terminology for the context you're describing would be the write buffer. Async writes will be stored in the ram write buffer and optimized for sequential disk blocks before writing to disk. Whenever there are sync writes, they will be written to the ZIL (hopefully you have a dedicated ZIL log device) immediately, and then they will join the write buffer with all the other async writes. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] ZFS and spindle speed (7.2k / 10k / 15k)
G'day All. I’m trying to select the appropriate disk spindle speed for a proposal and would welcome any experience and opinions (e.g. has anyone actively chosen 10k/15k drives for a new ZFS build and, if so, why?). This is for ZFS over NFS for VMWare storage ie. primarily random 4kB read/sync writes (SLOG) + some general CIFS file serving. About 40/60 read/write ratio. The primary drive options I’m trying to compare are 48xHP SAS 500gb 7.2k(avg 8ms seek - approx 80 random IOPS/drive) or 24xHP SAS 450g or 600g 10k drives (avg 4ms seek - approx 138 random IOPS/drive) which work out pretty close in price. Ok, first theory. Assuming sequential writes, the 7200 drives should be up to 75% (at worst 80/138%) the IOPS of the 10k and with twice the number of spindles and a low latency ZIL SLOG that should give much better write performance**. Correct?What IOPS are people seeing from 7200 (approx 8ms avg seek) drives under mainly write loads? Random reads IOPS are about the same on both options in terms of £/Random IO so the only problem is higher latency for reads that miss the ARC/L2ARC and are serviced by the 7200’s (avg 12.3- max 25.8ms) which is slower than the 10k would be (avg 7ms – max 14ms). I’m currently planning 2x240GB L2ARC so hopefully we’ll be able to get a lot of the active read memory into cache and keep the latencies low. Any suggestions how to identify the amount of “working dataset” on windows/netapp etc? I note ZFSBuild said they’ld do their next build with 15k SAS but I couldn’t follow their logic. Anything else I’m missing. ** My understanding is that ZFS will adjust the amount of data accepted into each “transaction” (TXG) to ensure it can be written to disk in 5s.Async data will stay in ARC, Sync data will also go to ZIL or, if overthreshold, will go to disk and pointer to ZIL(on low latency SLOG) – ie. all writes apart from sync writes over threshold will be unaffected by disk write latency from a client perspective.Therefore if, for the same budget, 7200rpm can give you a higher iops, high latency disk whereas 10k gives you lower latency but lower iops, the 7200rpm system would end up providing highest write iops at lowest latency (due to SLOG). -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
