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OpenBSD General Other questions regarding OpenBSD which do not fit in any of the categories below. |
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Softupdates and Softraid
I have made transfer tests to a Softraid RAID1 discipline, and write speed does not change much with or without softupdates enabled in my system. Is it an expected outcome from the Softraid overhead? Do I get enhanced data integrity from enabling Softupdates due to the ordered metadata?
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I understand.
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What could mean to the performance of the RAID1 that the time spent in disk accesses (by systat) are very different (eg, sd0 = 0.4, sd1 = 1.0)? sd0 is a Samsung and sd1 is a Seagate purchased in 2009 and 2012.
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Without knowing any of the details of your environment: disk drive models, hardware configuration (e.g.: shared bus or separate buses), competing uses of the drives outside of RAID, the details of the array's configuration, and a complete I/O performance report ... I can only make guesses. Here are three of them:
Guess 1: Perhaps sd1 is being utilized for reads more often than sd0 is? Guess 2: Perhaps you have active non-RAID partitions using sd1? Guess 3: Perhaps there are significant performance differences between the two disk drives? Your disk drive models and hardware configuration are in your dmesg(8). If you use either of these drives outside of the softraid(4) array, this should be clear from the drives' disklabel(8). There's a lot of information in the iostat view of systat(8): Code:
iostat Display statistics about disk throughput. Statistics on disk throughput show, for each drive, data transferred in kilobytes, number of disk transactions performed, and time spent in disk accesses (in fractions of a second). |
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Curiously, in read operations, sd0 is doubles the time spent by sd1; however in write operations is the opposite: sd1 doubles or triples the time spent by sd0.
BTW, I see a 50% penalty in read performance compared to not using softraid raid1. I mean, each disk alone has about 120 MB/s (non part of sr raid1), but RAID1 has a maximum of about 60 MB/s, giving each about 30 MB/s. Last edited by Martillo; 31st October 2013 at 03:35 PM. |
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Sorry, the read performance is about 100 MB/s using dd, with 50 MB/s each. The former 60 MB/s was surely interfered by qemu operations.
Do not worry jggimi, I will dig myself into the subject. In the end I am the one that has the keyboard attached to that machine and can experiment at will P.S.: Yes, I *like* machines. |
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Quote:
You may or may not be using a raw device with dd(1). You haven't stated. But you should, as block mode disk device nodes should not be used with dd on OpenBSD. They work, eventually, with significantly degraded performance. |
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So I should not use something like this
Code:
dd if=/dev/zero of=cero bs=1m count=1024 |
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No that's fine. I meant a read or write test to the underlying softraid device with dd. Block devices (e.g. /dev/sd4c) should not be used with dd. Instead, use the raw device (e.g. /dev/rsd4c).
This gives me the opportunity to state this again, for the fourth time. Complete information from you would go a long way towards having me providing you with any useful advice. |
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Sorry jggimi, I am in the process to upgrade to 5.4 (plus family duties). I am retaking the subject after the upgrade if you don't mind.
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Martillo, there was a question about RAID1 I/O on the OpenBSD misc@ mailing list today, that was just answered by Joel Sing, one of the lead developers on softraid(4).
http://marc.info/?l=openbsd-misc&m=138383080931098&w=2 |
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Thanks jggimi. I suspect that being both sdX disks on the same controller (X58 / ICH10R), the read may be somehow "limited", but it shouldn't be so much, so I suspect on the drives.
Both drives have 1 TB, but from different brands. I have tested them under Softraid, Raidctl and ZFS, and under ZFS the Samsung always gives bursts and idles intermittently while the Seagate has a steady read throughput, so I suspect he Seagate is lagging the Samsung. This leads me to the conclusion that in my specific case I get little benefit from RAID1. This machine is not a server, and I also have three different back up copies of my information in external USB and network devices, so no need for mirroring if it decreases my speed. |
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There was some follow-on discussion on that thread. Constantine Murenin noted that write speed will be at the speed of the slowest drive. However, as you are using the same controller, your I/Os are in sequence. SATA may be faster than PATA, but unless I misunderstand the technology -- and I might -- only one data transfer at a time is possible on a single ATA bus.
Last edited by jggimi; 7th November 2013 at 06:50 PM. Reason: typo |
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I have done some testing:
Common data: Commands to write & to read: Code:
write: dd if=/dev/urandom of= alea bs=10m count=1024 read: dd if=alea of=/dev/null disk1: Note: the u in urandom means the two different forms used in each OS. As we can see, the SAMSUNG disk performance surpasses the SEAGATE disk's one by factors of 2.1X (write) and 1.4X (read). Although they may work together, it is a waste for the faster to pair them. I also suspect that the slower suffers to keep up, so any kind of RAID with them is discarded. |
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