傳統磁盤本質上一種機械裝置,如FC, SAS, SATA磁盤,轉速通常爲5400/7200/10K/15K rpm不等。影響磁盤的關鍵因素是磁盤服務時間,即磁盤完成一個I/O請求所花費的時間,它由尋道時間、旋轉延遲和數據傳輸時間三部分構成。
尋道時間Tseek是指將讀寫磁頭移動至正確的磁道上所需要的時間。尋道時間越短,I/O操作越快,目前磁盤的平均尋道時間一般在3-15ms。
旋轉延遲Trotation是指盤片旋轉將請求數據所在扇區移至讀寫磁頭下方所需要的時間。旋轉延遲取決於磁盤轉速,通常使用磁盤旋轉一週所需時間的1/2表示。比如,7200 rpm的磁盤平均旋轉延遲大約爲60*1000/7200/2 = 4.17ms,而轉速爲15000 rpm的磁盤其平均旋轉延遲約爲2ms。
數據傳輸時間Ttransfer是指完成傳輸所請求的數據所需要的時間,它取決於數據傳輸率,其值等於數據大小除以數據傳輸率。目前IDE/ATA能達到133MB/s,SATA II可達到300MB/s的接口數據傳輸率,數據傳輸時間通常遠小於前兩部分時間。
因此,理論上可以計算出磁盤的最大IOPS,即IOPS = 1000 ms/ (Tseek + Troatation),忽略數據傳輸時間。假設磁盤平均物理尋道時間爲3ms, 磁盤轉速爲7200,10K,15K rpm,則磁盤IOPS理論最大值分別爲,
IOPS = 1000 / (3 + 60000/7200/2) = 140
IOPS = 1000 / (3 + 60000/10000/2) = 167
IOPS = 1000 / (3 + 60000/15000/2) = 200
固態硬盤SSD是一種電子裝置, 避免了傳統磁盤在尋道和旋轉上的時間花費,存儲單元尋址開銷大大降低,因此IOPS可以非常高,能夠達到數萬甚至數十萬。實際測量中,IOPS數值會受到很多因素的影響,包括I/O負載特徵(讀寫比例,順序和隨機,工作線程數,隊列深度,數據記錄大小)、系統配置、操作系統、磁盤驅動等等。因此對比測量磁盤IOPS時,必須在同樣的測試基準下進行,即便如何也會產生一定的隨機不確定性。通常情況下,IOPS可細分爲如下幾個指標:
Toatal IOPS,混合讀寫和順序隨機I/O負載情況下的磁盤IOPS,這個與實際I/O情況最爲相符,大多數應用關注此指標。
Random Read IOPS,100%隨機讀負載情況下的IOPS。
Random Write IOPS,100%隨機寫負載情況下的IOPS。
Sequential Read IOPS,100%順序負載讀情況下的IOPS。
Sequential Write IOPS,100%順序寫負載情況下的IOPS。
IOPS的測試benchmark工具主要有Iometer, IoZone, FIO等,可以綜合用於測試磁盤在不同情形下的IOPS。對於應用系統,需要首先確定數據的負載特徵,然後選擇合理的IOPS指標進行測量和對比分析,據此選擇合適的存儲介質和軟件系統。下面的磁盤IOPS數據來自http://en.wikipedia.org/wiki/IOPS,給大家一個基本參考。
Device | IOPS | Interface | Notes |
---|---|---|---|
7200 RPM SATA drives | ~90 IOPS | SATA II | |
10k RPM SATA drives, queue depth 1 | ~130 IOPS | SATA II | fio -readonly -name iops -rw=randread -bs=512 -runtime=20 -iodepth 1 -filename /dev/sda -ioengine libaio -direct=1 |
10k RPM Serial Attached SCSI drives | ~140 IOPS | SAS | |
15k RPM Serial Attached SCSI drives | ~180 IOPS | SAS | |
10k RPM SATA drives, queue depth 24 | ~290 IOPS | SATA II | fio -readonly -name iops -rw=randread -bs=512 -runtime=20 -iodepth 24 -filename /dev/sda -ioengine libaio -direct=1 |
Simple SLC SSD | ~400 IOPS | SATA II | |
Intel X25-M G2 (MLC; As of March 2010) | ~8,600 IOPS | SATA II | Intel's data sheet claims 6,600/8,600 IOPS (80GB/160GB version) and 35,000 IOPS for random 4KB writes and reads, respectively. |
Intel X25-E (SLC; As of March 2009) | ~5,000 IOPS | SATA II | Intel's data sheet claims 3,300 IOPS and 35,000 IOPS for writes and reads, respectively. 5,000 IOPS are measured for a mix. Intel X25-E G1 has around 3 times higher IOPS compared to the Intel X25-M G2. |
OCZ Z-Drive e84, a PCI Express SLC Solid State Drive | 16,000 IOPS | PCIe | Maximum read/write speed of 800/750 MB/s). |
G.Skill Phoenix Pro et. al. | ~20,000 IOPS | SATA III | SandForce-1200 based SSD drives with enhanced firmware, states up to 50,000 IOPS, but real-world performance shows for all drives from different manufacturers ~25,000 IOPS for random read and ~15,000 IOPS for random write. |
ioDrive, a PCI-Express card with Flash | 140,000 Read IOPS, 135,000 Write IOPS | PCIe | |
ioDrive Duo | 250,000+ IOPS | PCIe | |
DDRdrive X1, a May 2009 PCI Express based solid-state drive | 300,000+ (512B Random Read IOPS) and 200,000+ (512B Random Write IOPS) | PCIe | |
Texas Memory System
RamSan-20 |
120,000+ Random Read/Write IOPS | PCIe | Inlude RAM cache and UltraCapacitor to preserve the memory non-volatile |
Texas Memory System
RamSan-630 Appliance |
1,000,000+ Random Read/Write IOPS | FC / InfiniBand | |
Violin Memory Appliance | 1,000,000+ Random Read/Write IOPS | Proprietary interconnect | |
ioDrive Octal (single PCI Express card) | 1,000,000+ Random Read/Write IOPS | PCIe |