How much faster is a server equipped with a SSD than a mechanical hard disk

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I believe you all know that SSD （SSD） The advantage of this is that it is faster than the traditional mechanical hard disk （HDD） Be quick , So now more and more solid-state drives appear in online servers . But as a development that cares about performance numbers , I want to be more precise about carrying SSD The server is in IO The performance is better than that of carrying HDD How much faster , The order IO How much faster in the case of , Random IO How much faster under the circumstances ？ Finally, I took time to do a performance test recently .

Test environment

I found two servers , The disks are all Raid array , They are carrying SSD and HDD, More detailed configuration is as follows ：

• HDD Real machine ：
  • Raid The card model is PERC H730 Mini,1GB RAM cache
  • 4 block 500GB SSD Composed of raid5, Available capacity 1.3T
• SSD Real machine ：
  • Raid The card model is the same as above
  • 7 block 300G HDD The hard disk makes up Raid5, Available capacity 1.6T

Okay , The contestants are in position , Then select the pressure measuring tool fio. Designated ratio ( Pressure ) " ( measuring ) gauge ( ginseng ) be ( Count ) as follows ：

• IO engine ： In order to put enough pressure on the hard disk , Choose asynchronous mode libaio
• PageCache： In order to get closer to the performance of the disk itself , Turn on Direct Parameters bypass Linux System memory cache
• Pressure test object ： On the safe side , No naked device used , The files used
• file size ：100GB, Because of my Raid The card cache is 1G, The guarantee is much bigger than the cache
• Scheduling strategy ： Choose common noop

The contrast test officially began .

Sequential read PK

Let's start with a test under sequential access , Because in this environment, whether it's a mechanical hard disk , Or a solid-state drive , Performance will be optimal .IO Size from 512K Until 1M.

Let's take a look at the mechanical hard disk ：

Under SSD ：

The data looks like , In order IO Under the circumstances , Actually, it carries HDD Server for IO Performance is not being carried ssd It's falling too much . This is because

• The order IO Under the circumstances ,RAID The card has a high hit rate , Especially when it's set up RAID Prefetch
• In its own order IO It's also the best state of mechanical disk work

therefore , Modern in the field of servers ,SSD It's just that there are more and more applications , But it doesn't replace HDD. One of the reasons is because of the order of the mechanical hard disks IO It's not slow , Plus Raid Blessing , So there's a big market share . After comparing the order IO, Let's compare another application scenario , Random IO.

Random reads PK

Remember the beginning fio In the pressure measurement parameters, we put Page Cache It's disabled , Plus the size of the test file is 100GB, This is much more than Raid The cache size of the card . So random based on this premise IO, All levels of cache are basically out of use ,IO Performance speed depends entirely on the hard disk itself .

stay 《 Mechanical hard disk random IO Slower than you think 》 in , We look at the random of mechanical hard disks IO It's really bad performance .

Let's look at carrying SSD Server for IO performance ：

By contrast , Mechanical hard disk random IO It's not that slow , We choose the most recognized result of the above 4K IO size Look at .

• From a bandwidth perspective , Only less than 1M The throughput of ,
• From the perspective of delay , stay 4.4ms about

Let's look at the SSD 4K IO Size Next performance . And under the same conditions SSD Can have 32M.

• From a bandwidth perspective , There are still 32M, Than HDD It can be dozens of times faster
• From the perspective of delay , about 120us about . Than HDD, Namely 4400：120, It's also dozens of times higher

summary

In order IO Under the circumstances , Because there is Raid The blessing of card cache , be based on HDD Composed of Raid The array server is in IO The performance is not too bad . But in a random read scenario , Whether bandwidth or latency metrics ,SSD It's better than HDD More than ten times faster . So in the server application SSD, The main solution is random IO The problem of . So if you have too many Page Cache,Raid Card cache can't hold random IO request , Then please put HDD Replace it with SSD Well .

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Development of hard disk album of internal training ：

• 1. Disk opening ： Take off the hard coat of the mechanical hard disk ！
• 2. Disk partitioning also implies technical skills
• 3. How can we solve the problem that mechanical hard disks are slow and easy to break down ？
• 4. Disassemble the SSD structure
• 5. How much disk space does a new empty file take ？
• 6. Only 1 How much disk space does a byte file actually take up
• 7. When there are too many documents ls Why is the command stuck ？
• 8. Understand the principle of formatting
• 9.read How much disk does a byte of file actually take place on IO？
• 10.write When to write to disk after one byte of file IO？
• 11. Mechanical hard disk random IO Slower than you think
• 12. How much faster is a server equipped with a SSD than a mechanical hard disk ？

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