Disk Performance Hands On, Part 4: RAID 1 Performance

<div><p><em>This is Part 4 in an ongoing series on disk performance. You can read the entire series by starting at the Introduction.</em></p> <p>RAID 1 is typically recommended for transaction log files, though I have known several people (including myself, I’ll admit) who have also used RAID 1 to hold data files. Unlike RAID 5 & 10 there is no stripe size to factor into performance since RAID 1 only utilizes mirroring. Still, it’s important to use the right offset and allocation unit size - or is it? For these tests I used Test Harness #1 (Dell PowerEdge 2950) and Test Harness #3 (Dell PowerVault MD1000). You can view the test harness specifics here.</p> <p><strong>Partition Offset & Allocation Unit Size - Dell PowerEdge 2950 <br/></strong>Based on the results from my RAID 10 and RAID 5 tests I chose to focus on a 64 KB allocation unit. In addition to 32 KB and 64 KB offset values I also tested a 1024 KB offset to see if a larger offset made a difference. As with previous tests I’ll present the results for 8 KB random reads and writes, the two operations SQL performs on data files most of the time.</p> <p>Here’s what 8 KB random reads look like:</p> <p><img alt="IOs/sec, 8 KB random reads, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEiULwlAuQsg9FwYLlD_JQ4zpB-lC3bc5cN6eDa7iEDXyfh5_2_EWQb1BTZFv6tMpDWpmujXDTVSbCNqpyvk58yWkQz5t5gAAKOI5vfESE5cM2LnqesxxZNBWXOSE28f8LYfX-At2SIJcpP1/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="IOs/sec, 8 KB random reads, 32\64\1024 KB offset" width="483"/> <img alt="MBs/sec, 8 KB random reads, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEh9kyt69sL09sOxtmNSIzOWWO1ETqkWhZuFLgnp3ZZvFqobcNhErwUYWdq2doR9x4u82dW3f97uGh4T_u8wNGHHD6hfXTYzyEuWr3OS39f8PxLgeKMuXKIkL1JulbQscqkDOXyBMvPTAIFY/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="MBs/sec, 8 KB random reads, 32\64\1024 KB offset" width="483"/><img alt="Avg Latency, 8 KB random reads, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEhtkgW4CVGEkxQRxCrhEDs46Qxw8jBa1yVtZEovUR-F3UCuyKSMiyWus693_MumUfTPMLjob26xZ7l4CxjJT424UrmDodIl-Vkhn5GFtLLbFwJc45P3jUcRvIbhYX0zuvPTJaoH4lQ6Mc9t/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="Avg Latency, 8 KB random reads, 32\64\1024 KB offset" width="483"/></p> <p>And here’s what 8 KB random writes look like:</p> <p><img alt="IOs/sec, 8 KB random writes, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEiJREfbR7hKbSkM-aiIUyx-RlS0rIKy1ZFOCx8jFK0c-LjmqM3Zw2tIBaC7PFm0cThhZV1e6n_OcD_6-c4sAQQBkMWFdepF1toEuRQydStzAR0aHSmM0Owr9JYODDAsH8LWCy_vMczP58yX/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="IOs/sec, 8 KB random writes, 32\64\1024 KB offset" width="483"/> <img alt="MBs/sec, 8 KB random writes, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEiVqbbVK1oLb4ZSIxQ0BkhaHpawmvIrr15HpXXA5HQQ2HTCoG1beXPZzY8BwjeU2pJ0DAKUYI1M_vq4Ahh4ygKKun30mt24BnhM_wp3l8IEFliOQ4BcftwkvoJ5g9lY4DTHD7BCF_f-qsm2/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="MBs/sec, 8 KB random writes, 32\64\1024 KB offset" width="483"/> <img alt="Avg Latency, 8 KB random writes, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEg1VjrmEVOHJ2Lzn6a1yxAzP3rkMEwxFoExNYN7NVcrvF_oRP7O7N0pOeLo5Dq7dkWwU55XKRR-OWyCisVMFAq15sVEIgD0sUu9YGCMXhkvuPMbEh3ib3RS-UTFE7LE4xIVZatMAmGkx3p5/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="Avg Latency, 8 KB random writes, 32\64\1024 KB offset" width="483"/></p> <p>For both reads and writes the numbers are so close together that there’s no practical difference between them. I’ll point out that Test Harness #1 was using (somewhat) older 72 GB 15K RPM <a href="http://en.wikipedia.org/wiki/SCSI" target="_blank">SCSI</a> drives. Let’s take a look at a slightly newer system that uses 146 GB 15K RPM <a href="http://en.wikipedia.org/wiki/Serial_Attached_SCSI" target="_blank">SAS</a> drives.</p> <p><strong>Partition Offset & Allocation Unit Size - Dell PowerVault MD1000: <br/></strong>Let’s look at the same tests against a Dell PowerVault MD1000 (Test Harness #3). I’m also going to add one more thing to the mix - in my tests against the PowerEdge 2950 I configured SQLIO to use 4 threads. To see if the results were the same across increased workloads I also ran my tests using 8 and 16 threads.</p> <p>Here’s what 8 KB random reads look like:</p> <p><img alt="IOs/sec, 8 KB random reads, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEjLzn12nAqnrqVGAE7_WUWpPOYmRg6J4VMuKwj0iJ6i32Y2yDa8hhLVcHk9CQeI-xlrsFdu9jSGGS6NfYwpMJFbuI_gLZf_jxxHNVMRnX-8XyZuVcKZWxl9djuZPLEeMqSFxkhZPTJXliBz/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="IOs/sec, 8 KB random reads, 32\64\1024 KB offset" width="483"/> <img alt="MBs/sec, 8 KB random reads, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEi6mGkiH4Q_C4rs5wqjYcIXB3HS-CYZyXUXWOpnCw0FDHeU6MM1qOHI_A2lNCXwGt8DMrMSAOevVaZqWc7N0-PAsYihcuV3bJj0PnL2uu3H5y3XRylGQqwnxN80nbLDwrrlTgimLP2YvX85/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="MBs/sec, 8 KB random reads, 32\64\1024 KB offset" width="483"/> <img alt="Avg Latency, 8 KB random reads, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEidnqBvtxMIOXBwmwFaE8cLO2amueGmzGcrK83WOXwLqhUdJ-tvtrAQXdVFUPvvtsb6G8tHVYt-x2d2WwH0zbc6JxPIGJooonv6l06CpUYwC5e7w3-rkQNMmSTUGpzKanV1Sn4fJc2_MU09/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="Avg Latency, 8 KB random reads, 32\64\1024 KB offset" width="483"/> </p> <p>And here’s what 8 KB random writes look like: </p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p></p> <p><img alt="IOs/sec, 8 KB random writes, 32\64\1024 KB offset" border="0" height="291" src="../../images/AVvXsEhI5cOf_fklULM-m7EGKIG3RPOyi46jXEOYQANkysNyAcodnCaujahHyvSx_f89d4JIalKhyphenhyphenqh6kyXM20MycOn2jcK0FU8W06xaoSuMrjP8q_U5a_xEjhlVd5tH-Y0HvC1pIhzdwZEg9VCO/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="IOs/sec, 8 KB random writes, 32\64\1024 KB offset" width="483"/> <img alt="MBs/sec, 8 KB random writes, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEjXdPZbYyFD8W486e_pgLOJv6umw_YRfYg0qDXj2-rwumWBe37icQ3QgOOHw_pI9pu5cNkoGb0x8LPO6FPSEs866K7dJckbd_IvQVaCAGqT-wntJLoDrB8igKPX1P59qVXVVZ6V18ZF-LMh/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="MBs/sec, 8 KB random writes, 32\64\1024 KB offset" width="483"/> <img alt="Avg Latency, 8 KB random writes, 32\64\1024 KB offset" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEhd7kwa0-DwwAVhEBkVDF6y2PXVta0cJOnxOoD56qKDuIzxCmHp23I5iKxCUDZDPphjVHhKXd3o9jgo2yk8pRIKXfkcxC5QugZ9rgS0oeCu5fmU4_ZVkQqtKJEt99Z8HjZ6X9LfISnp7P6f/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="Avg Latency, 8 KB random writes, 32\64\1024 KB offset" width="483"/> </p> <p>Now we see at least a minimal difference in the numbers and it appears that using a 64 KB offset or higher results in a small increase in performance. Just how much? For 64 KB vs. 32 KB offset it’s a ~10% improvement in IOs/sec, a ~5% improvement in MBs/sec, and a ~10% improvement in average latency for 8 KB random reads. For 8 KB random writes improvements are ~10.5% in IOs/sec, ~11% in MBs/sec, and 10-11% in average latency. Going from a 64 KB offset to a 1024 KB offset resulted in a very small improvement (about 1%), but an improvement nonetheless.</p> <p><strong></strong></p> <p><strong>Transaction Log Write Performance - Dell PowerEdge 2950 <br/></strong>Back to Test Harness #1 for a second. Remember that RAID 1 drives are typically used for transaction logs. Unlike data reads\writes, transaction log writes are single threaded, sequential operations that range between 512 bytes and 64 KB. To see how performance varied for this kind of activity I ran a set of tests on a 2 GB file using 8 KB and 64 KB writes on 4 KB, 8 KB, and 64 KB allocation units on both 32 KB and 64 KB offsets.</p> <p>Here’s what sequential writes look like for a 32 KB offset:</p> <p><img alt="IOs/sec, 8\64 KB sequential writes, 64 KB offset, 4\8\64 KB allocation unit" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEj3kfTDjMHTTKjoC4poBtCyHNTy-hX68OOOEzmutwQ23QDBd-oFxKcjeIx3lHMhcN8yX_nYFCXefsI9Fp5dGQ97g1UFjO7diGSfPLFGF9oeFUJJ9jIMASP-dGpTojkSPLrWGYZj-m2O5lH3/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="IOs/sec, 8\64 KB sequential writes, 64 KB offset, 4\8\64 KB allocation unit" width="483"/> <img alt="MBs/sec, 8\64 KB sequential writes, 64 KB offset, 4\8\64 KB allocation unit" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEgmGt1XbuoXBHdD5k42pM_bI_4Vo0FUVKuE-FlaujcXigEHZByv7O5gNKbHTJrMRHRX_ivY64_bGRiRYPyJ4sAAjW5HXtyhH8KfSmgvJAaZKsrbnZLXAkzvXlNi9F5cnrByCI1s9T8ghv8o/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="MBs/sec, 8\64 KB sequential writes, 64 KB offset, 4\8\64 KB allocation unit" width="479"/> <img alt="image" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEiE5O-JDSGL0JFI0tfboORQrJ6blSHHVQJ8pyKz7moJ8i7JXoEtPN8CCArNs61f8b0IdAtVfUXeCNZk-7UGGQQM_GUlH7Ft09f_3DMN4knxLN8m8q3QSwW6S2XtfXgU0b4GktkRvxu1dcX1/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="image" width="480"/> </p> <p>And here’s what sequential writes look like for a 64 KB offset:</p> <p><img alt="image" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEhWBR4ChZuUjfbA5E3QeHbkwgF4GZsrO9YweXFRcj372r5rmELB4hFmE7ZK-uPjzq-I0EmKf9NHAApiISu6UQ55Rz-mKonG64nj0UeKP0GnLg_pehicVOhp44fV69rJiHLmokEdImJCU-4B/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="image" width="483"/> <img alt="image" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEgeKTxjLTKIBBhkMxljzuGpttvHhBpYjTrCd8KApXjdJ_eKyu72Ii52U5gRtgd2FjiwA6j_jcgR9KVTF4HZchyfHTRj5KA_yUa3ryISonVYG1j89bkFVXdoT120fUfh_pPqQgLVYht7NERE/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="image" width="479"/> <img alt="Avg Latency, 8\64 KB sequential writes, 64 KB offset, 4\8\64 KB allocation unit" border="0" height="291" src="../../img/b/R29vZ2xl/AVvXsEjhqAllzF8M7g2QjQSoiPyIh7Jy6fURYdC_Nc8Xf9zprK_gKb5eAWqFDmlQmezfASAZb5II3sgn8ZTYgtMixHuyKr5nHNRpjkverS3N3auyWHkfpYG6boTUADao9oQXg_FYUC4lar6VKq7m/-imgmax-800.jpg" style="border-top-width: 0px; display: inline; border-left-width: 0px; border-bottom-width: 0px; border-right-width: 0px" title="Avg Latency, 8\64 KB sequential writes, 64 KB offset, 4\8\64 KB allocation unit" width="480"/> <br/>I’m not going to bother to show any comparisons of 64 KB vs. 32 KB because the numbers are pretty much dead-on the same. Would there be any difference on newer hardware? Perhaps, but my guess is that it wouldn’t be anything dramatic.</p> <p><strong>Conclusion <br/></strong>For OLTP data files on RAID 1 my tests showed the optimal configuration to be a 64 KB allocation unit and at least a 64 KB offset. For OLTP transaction log files on RAID 1 it doesn’t matter what offset or allocation unit size you use, there doesn’t appear to be any combination of settings that result in a significant performance improvement.</p> <p>In Part 5 I’ll take a look at how RAID 10 performance stacks up against RAID 5.</p> </div>

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