Thread: H/W RAID 5 on slower disks versus no raid on faster HDDs
Hi folks, I have two options: 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI controller + H/W Raid 5 and 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID Does anyone opinions *performance wise* the pros and cons of above two options. please take in consideration in latter case its higher RPM and better SCSI interface. Regds Mallah. -- Rajesh Kumar Mallah, Project Manager (Development) Infocom Network Limited, New Delhi phone: +91(11)6152172 (221) (L) ,9811255597 (M) Visit http://www.trade-india.com , India's Leading B2B eMarketplace.
How are you going to make use of the three faster drives under postgresql? Were you intending to put the WAL, system/swap, and the actual data files on separate drives/partitions? Unless you do something like that (or s/w RAID to distribute the processing across the disks), you really have ONE SCSI 15K Ultra320 drive against 3 slower drives with the RAID overhead (and spreading of performance because of the multiple heads). I don't have specifics here, but I'd expect that the RAID5 on slower drives would work better for apps with lots of selects or lots of concurrent users. I suspect that the Ultra320 would be better for batch jobs and mostly transactions with less selects. Charlie Rajesh Kumar Mallah. wrote: >Hi folks, > >I have two options: >3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI controller + H/W Raid 5 >and >2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID > >Does anyone opinions *performance wise* the pros and cons of above >two options. > >please take in consideration in latter case its higher RPM and better >SCSI interface. > > > >Regds >Mallah. > > > > > > -- Charles H. Woloszynski ClearMetrix, Inc. 115 Research Drive Bethlehem, PA 18015 tel: 610-419-2210 x400 fax: 240-371-3256 web: www.clearmetrix.com
On Thu, 21 Nov 2002, Rajesh Kumar Mallah. wrote: > > Hi folks, > > I have two options: > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI controller + H/W Raid 5 > and > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID > > Does anyone opinions *performance wise* the pros and cons of above > two options. > > please take in consideration in latter case its higher RPM and better > SCSI interface. Does the OS you're running on support software RAID? If so the dual 36 gigs in a RAID0 software would be fastest, and in a RAID1 would still be pretty fast plus they would be redundant. Depending on your queries, there may not be a lot of difference between running the 3*18 hw RAID or the 2*36 setup, especially if most of your data can fit into memory on the server. Generally, the 2*36 should be faster for writing, and the 3*18 should be about even for reads, maybe a little faster.
Oh i did not mention, its linux, it does. RAM: 2.0 GB CPU: Dual 2.0 Ghz Intel Xeon DP Processors. On Thursday 21 November 2002 23:02, scott.marlowe wrote: > On Thu, 21 Nov 2002, Rajesh Kumar Mallah. wrote: > > Hi folks, > > > > I have two options: > > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI controller + H/W Raid 5 > > and > > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID > > > > Does anyone opinions *performance wise* the pros and cons of above > > two options. > > > > please take in consideration in latter case its higher RPM and better > > SCSI interface. > > Does the OS you're running on support software RAID? If so the dual 36 > gigs in a RAID0 software would be fastest, and in a RAID1 would still be > pretty fast plus they would be redundant. > > Depending on your queries, there may not be a lot of difference between > running the 3*18 hw RAID or the 2*36 setup, especially if most of your > data can fit into memory on the server. > Generally, the 2*36 should be faster for writing, and the 3*18 should be > about even for reads, maybe a little faster. Since i got lots of RAM and my Data Size (on disk ) is 2 GB i feel frequent reads can happen from the memory. I have heard putting pg_xlog in a drive of its own helps in boosting updates to DB server. in that case shud i forget abt the h/w and use one disk exclusively for the WAL? Regds mallah. -- Rajesh Kumar Mallah, Project Manager (Development) Infocom Network Limited, New Delhi phone: +91(11)6152172 (221) (L) ,9811255597 (M) Visit http://www.trade-india.com , India's Leading B2B eMarketplace.
OK now i am reading Momjian's "PostgreSQL Hardware Performance Tuning" once again ;-) mallah. On Thursday 21 November 2002 23:02, scott.marlowe wrote: > On Thu, 21 Nov 2002, Rajesh Kumar Mallah. wrote: > > Hi folks, > > > > I have two options: > > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI controller + H/W Raid 5 > > and > > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID > > > > Does anyone opinions *performance wise* the pros and cons of above > > two options. > > > > please take in consideration in latter case its higher RPM and better > > SCSI interface. > > Does the OS you're running on support software RAID? If so the dual 36 > gigs in a RAID0 software would be fastest, and in a RAID1 would still be > pretty fast plus they would be redundant. > > Depending on your queries, there may not be a lot of difference between > running the 3*18 hw RAID or the 2*36 setup, especially if most of your > data can fit into memory on the server. > > Generally, the 2*36 should be faster for writing, and the 3*18 should be > about even for reads, maybe a little faster. -- Rajesh Kumar Mallah, Project Manager (Development) Infocom Network Limited, New Delhi phone: +91(11)6152172 (221) (L) ,9811255597 (M) Visit http://www.trade-india.com , India's Leading B2B eMarketplace.
I had long labored under the impression that RAID 5 should give me better performance but I have since encountered many reports that this is not the case. Do some searching on Google and you will probably find numerous articles. Note 3x18 w/RAID5 will give 36GB usable while 2x36 w/o RAID is 72GB. You could use mirroring on the 2x36 and have the same usable space. A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a single disk) and an improvement on reads due to having two drives to read from and will (based on the Scientific Wild Ass Guess method and knowing nothing about your overall system) probably be faster than the RAID5 configuration while giving you identical usable space and data safety. You also may see improvements due to the 15,000RPM drives (of course RPM is sort of an arbitrary measure - you really want to know about track access times, latency, transfer rate, etc. and RPM is just one influencing factor for the above). The quality of your RAID cards will also be important (how fast do they perform their calculations, how much buffer do they have) as will the overall specs of you system. If you have a bottleneck somewhere other than your raw disk I/O then you can throw all the money you want at faster drives and see no improvement. Cheers, Steve On Thursday 21 November 2002 8:45 am, you wrote: > Hi folks, > > I have two options: > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI controller + H/W Raid 5 > and > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID > > Does anyone opinions *performance wise* the pros and cons of above > two options. > > please take in consideration in latter case its higher RPM and better > SCSI interface. > > > > Regds > Mallah.
Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on faster HDDs
From
"Rajesh Kumar Mallah."
Date:
Thanks Steve, recently i have come to know that i can only get 3*18 GB ultra160 10K hraddrives, my OS is lunux , other parameters are RAM:2GB , CPU:2*2Ghz Xeon, i feel i will do away with raid use one disk for the OS and pg_dumps , one for tables and last one for WAL , does this sound good? regds mallah. On Friday 22 November 2002 00:26, Steve Crawford wrote: > I had long labored under the impression that RAID 5 should give me better > performance but I have since encountered many reports that this is not the > case. Do some searching on Google and you will probably find numerous > articles. > > Note 3x18 w/RAID5 will give 36GB usable while 2x36 w/o RAID is 72GB. > You could use mirroring on the 2x36 and have the same usable space. > > A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a > single disk) and an improvement on reads due to having two drives to read > from and will (based on the Scientific Wild Ass Guess method and knowing > nothing about your overall system) probably be faster than the RAID5 > configuration while giving you identical usable space and data safety. > > You also may see improvements due to the 15,000RPM drives (of course RPM is > sort of an arbitrary measure - you really want to know about track access > times, latency, transfer rate, etc. and RPM is just one influencing factor > for the above). > > The quality of your RAID cards will also be important (how fast do they > perform their calculations, how much buffer do they have) as will the > overall specs of you system. If you have a bottleneck somewhere other than > your raw disk I/O then you can throw all the money you want at faster > drives and see no improvement. > > Cheers, > Steve > > On Thursday 21 November 2002 8:45 am, you wrote: > > Hi folks, > > > > I have two options: > > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI controller + H/W Raid 5 > > and > > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID > > > > Does anyone opinions *performance wise* the pros and cons of above > > two options. > > > > please take in consideration in latter case its higher RPM and better > > SCSI interface. > > > > > > > > Regds > > Mallah. > > ---------------------------(end of broadcast)--------------------------- > TIP 2: you can get off all lists at once with the unregister command > (send "unregister YourEmailAddressHere" to majordomo@postgresql.org) -- Rajesh Kumar Mallah, Project Manager (Development) Infocom Network Limited, New Delhi phone: +91(11)6152172 (221) (L) ,9811255597 (M) Visit http://www.trade-india.com , India's Leading B2B eMarketplace.
> A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a > single disk) and an improvement on reads due to having two drives to read > from and will (based on the Scientific Wild Ass Guess method and knowing slightly offtopic: Does anyone one if linux software raid 1 supports this method (reading from both disks, thus doubling performance)? Regards, Bjoern
> Does anyone one if linux software raid 1 supports this method (reading from > both disks, thus doubling performance)? >
On Fri, 22 Nov 2002, Rajesh Kumar Mallah. wrote: > > > Thanks Steve, > > recently i have come to know that i can only get 3*18 GB ultra160 10K > hraddrives, > > my OS is lunux , other parameters are > RAM:2GB , CPU:2*2Ghz Xeon, > > i feel i will do away with raid use one disk for the OS > and pg_dumps > > , one for tables and last one for WAL , does this sound good? That depends. Are you going to be mostly reading, mostly updating, or an even mix of both? If you are going to be 95% reading, then don't bother moving WAL to another drive, install the OS on the first 2 or 3 gigs of each drive, then make a RAID5 out of what's left over and put everything on that. If you're going to be mostly updating, then yes, your setup is a pretty good choice. If it will be mostly mixed, look at using a software RAID1. More important will be tuning your database once it's up, i.e. increasing shared buffers, setting random page costs to reflect what percentage of your dataset is likely to be cached (the closer you come to caching your whole dataset, the closer random page cost approaches 1)
Bjoern,
You may find that hoping for a doubling of performance by using RAID 1 is a little on the optimistic side.
Except on very long sequential reads, media transfer rates are unlikely to be the limiting factor in disk throughput. Seek and rotational latencies are the cost factor in random I/O, and with RAID 1, the performance gain comes from reducing the mean latency -- on a single request, one disk will be closer to the data than the other. If the software that's handling the RAID 1 will schedule concurrent requests, you lose the advantage of reducing mean latency in this fashion, but you can get some improvement in throughput by overlapping some latency periods.
While not wanting to argue against intelligent I/O design, memory is cheap these days, and usually gives big bang-for-buck in improving response times.
As to the specifics of how one level or another of Linux implements RAID 1, I'm afraid I can't shed much light at the moment.
Regards,
Mike
On Fri, 2002-11-22 at 06:24, Bjoern Metzdorf wrote:
You may find that hoping for a doubling of performance by using RAID 1 is a little on the optimistic side.
Except on very long sequential reads, media transfer rates are unlikely to be the limiting factor in disk throughput. Seek and rotational latencies are the cost factor in random I/O, and with RAID 1, the performance gain comes from reducing the mean latency -- on a single request, one disk will be closer to the data than the other. If the software that's handling the RAID 1 will schedule concurrent requests, you lose the advantage of reducing mean latency in this fashion, but you can get some improvement in throughput by overlapping some latency periods.
While not wanting to argue against intelligent I/O design, memory is cheap these days, and usually gives big bang-for-buck in improving response times.
As to the specifics of how one level or another of Linux implements RAID 1, I'm afraid I can't shed much light at the moment.
Regards,
Mike
On Fri, 2002-11-22 at 06:24, Bjoern Metzdorf wrote:
> A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a > single disk) and an improvement on reads due to having two drives to read > from and will (based on the Scientific Wild Ass Guess method and knowing slightly offtopic: Does anyone one if linux software raid 1 supports this method (reading from both disks, thus doubling performance)? Regards, Bjoern ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster
|
Michael Nielsen ph: 0411-097-023 email: miken@bigpond.net.au Mike Nielsen |
On Thu, 21 Nov 2002, Bjoern Metzdorf wrote: > > A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a > > single disk) and an improvement on reads due to having two drives to read > > from and will (based on the Scientific Wild Ass Guess method and knowing > > slightly offtopic: > > Does anyone one if linux software raid 1 supports this method (reading from > both disks, thus doubling performance)? Yes, it does. Generally speaking, it increases raw throughput by a factor of 2 if you're grabbing enough data to justify reading it from both drives. But for most database apps, you don't read enough at a time to get a gain from this. I.e. if your stripe size is 8k and you're reading 1k at a time, no gain. However, under parallel load, the extra drives really help. In fact, the linux kernel supports >2 drives in a mirror. Useful for a mostly read database that needs to handle lots of concurrent users.
> In fact, the linux kernel supports >2 drives in a mirror. Useful for a > mostly read database that needs to handle lots of concurrent users. Good to know. What do you think is faster: 3 drives in raid 1 or 3 drives in raid 5? Regards, Bjoern
Bjoern, > Good to know. > > What do you think is faster: 3 drives in raid 1 or 3 drives in raid > 5? My experience? Raid 1. But that depends on other factors as well; your controller (software controllers use system RAM and thus lower performance), what kind of reads you're getting and how often. IMHO, RAID 5 is faster for sequential reads (lareg numbers of records on clustered tables), RAID 1 for random reads. And keep in mind: RAID 5 is *bad* for data writes. In my experience, database data-write performance on RAID 5 UW SCSI is as slow as IDE drives, particulary for updating large numbers of records, *unless* the updated records are sequentially updated and clustered. But in a multi-user write-often setup, RAID 5 will slow you down and RAID 1 is better. Did that help? -Josh Berkus
Bjoern, > Good to know. > > What do you think is faster: 3 drives in raid 1 or 3 drives in raid > 5? My experience? Raid 1. But that depends on other factors as well; your controller (software controllers use system RAM and thus lower performance), what kind of reads you're getting and how often. IMHO, RAID 5 is faster for sequential reads (lareg numbers of records on clustered indexes), RAID 1 for random reads. And keep in mind: RAID 5 is *bad* for data writes. In my experience, database data-write performance on RAID 5 UW SCSI is as slow as IDE drives, particulary for updating large numbers of records, *unless* the updated records are sequentially updated and clustered. But in a multi-user write-often setup, RAID 5 will slow you down and RAID 1 is better. Did that help? -Josh Berkus
On Thu, 21 Nov 2002, Bjoern Metzdorf wrote: > > In fact, the linux kernel supports >2 drives in a mirror. Useful for a > > mostly read database that needs to handle lots of concurrent users. > > Good to know. > > What do you think is faster: 3 drives in raid 1 or 3 drives in raid 5? Generally RAID 5. RAID 1 is only faster if you are doing a lot of parellel reads. I.e. you have something like 10 agents reading at the same time. RAID 5 also works better under parallel load than a single drive. The fastest of course, is multidrive RAID0. But there's no redundancy. Oddly, my testing doesn't show any appreciable performance increase in linux by layering RAID5 or 1 over RAID0 or vice versa, something that is usually faster under most setups.
> Generally RAID 5. RAID 1 is only faster if you are doing a lot of > parellel reads. I.e. you have something like 10 agents reading at the > same time. RAID 5 also works better under parallel load than a single > drive. yep, but write performance sucks. > The fastest of course, is multidrive RAID0. But there's no redundancy. With 4 drives I'd always go for raid 10, fast and secure > Oddly, my testing doesn't show any appreciable performance increase in > linux by layering RAID5 or 1 over RAID0 or vice versa, something that > is usually faster under most setups. Is this with linux software raid? raid10 is not significantly faster? cant believe that... Regards, Bjoern
On Thu, 21 Nov 2002, Bjoern Metzdorf wrote: > > Generally RAID 5. RAID 1 is only faster if you are doing a lot of > > parellel reads. I.e. you have something like 10 agents reading at the > > same time. RAID 5 also works better under parallel load than a single > > drive. > > yep, but write performance sucks. Well, it's not all that bad. After all, you only have to read the parity stripe and data stripe (two reads) update the data stripe, xor the new data stripe against the old parity stripe, and write both. In RAID 1 you have to read the old data stripe, update it, and then write it to two drives. So, generally speaking, it's not that much more work on RAID 5 than 1. My experience has been that RAID5 is only about 10 to 20% percent slower than RAID1 in writing, if that. > > The fastest of course, is multidrive RAID0. But there's no redundancy. > > With 4 drives I'd always go for raid 10, fast and secure > > > Oddly, my testing doesn't show any appreciable performance increase in > > linux by layering RAID5 or 1 over RAID0 or vice versa, something that > > is usually faster under most setups. > > Is this with linux software raid? raid10 is not significantly faster? cant > believe that... Yep, Linux software raid. It seems like it doesn't parallelize well. That's with several different setups. I've tested it on a machine a dual Ultra 40/80 controller and 6 Ultra wide 10krpm SCSI drives, and no matter how I arrange the drives, 50, 10, 01, 05, the old 1 or 5 setups are just about as fast.
Am Donnerstag, 21. November 2002 21:53 schrieb Bjoern Metzdorf:
> > In fact, the linux kernel supports >2 drives in a mirror. Useful for a
> > mostly read database that needs to handle lots of concurrent users.
>
> Good to know.
>
> What do you think is faster: 3 drives in raid 1 or 3 drives in raid 5?
>
> Regards,
> Bjoern
>
If 4 drives are an option, I suggest 2 x RAID1, one for data, and one for WAL and temporary DB space (pg_temp).
Regards,
Mario Weilguni
Mario Weilguni <mweilguni@sime.com> writes:
> If 4 drives are an option, I suggest 2 x RAID1, one for data, and one for WAL and temporary DB space (pg_temp).
Ideally there should be *nothing* on the WAL drive except WAL; you don't
ever want that disk head seeking away from the WAL. Put the temp files
on the data disk.
regards, tom lane
pgsql-performance-owner@postgresql.org wrote: > Objet : Re: [PERFORM] [ADMIN] H/W RAID 5 on slower disks versus no > raid on > > > Mario Weilguni <mweilguni@sime.com> writes: >> If 4 drives are an option, I suggest 2 x RAID1, one for data, and >> one for WAL and temporary DB space (pg_temp). > > Ideally there should be *nothing* on the WAL drive except WAL; you > don't ever want that disk head seeking away from the WAL. Put the > temp files on the data disk. > > regards, tom lane > > ---------------------------(end of > broadcast)--------------------------- TIP 4: Don't 'kill -9' the > postmaster which temp files ?
On Fri, Nov 22, 2002 at 08:52:48AM -0500, Tom Lane wrote:
> Mario Weilguni <mweilguni@sime.com> writes:
> > If 4 drives are an option, I suggest 2 x RAID1, one for data, and one for WAL and temporary DB space (pg_temp).
>
> Ideally there should be *nothing* on the WAL drive except WAL; you don't
> ever want that disk head seeking away from the WAL. Put the temp files
> on the data disk.
Unless the interface and disks are so fast that it makes no
difference.
Try as I might, I can't make WAL go any faster on its own controller
and disks than if I leave it on the same filesystem as everything
else, on our production arrays. We use Sun A5200s, and I have tried
it set up with the WAL on separate disks on the box, and on separate
disks in the array, and even on separate disks on a separate
controller in the array (I've never tried it with two arrays, but I
don't have infinite money, either). I have never managed to
demonstrate a throughput difference outside the margin of error of my
tests. One arrangement -- putting the WAL on a separate pair of UFS
disks using RAID 1, but not on the fibre channel -- was demonstrably
slower than leaving the WAL in the data area.
Nothing is proved by this, of course, except that if you're going to
use high-performance hardware, you have to tune and test over and
over again. I was truly surprised that a separate pair of VxFS
RAID-1 disks in the array were no faster, but I guess it makes sense:
the array is just as busy in either case, and the disks are really
fast. I still don't really believe it, though.
A
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