Write caching improves server performance

With computing demands continually increasing, there is a constant need for speed when it comes to accessing disk storage. In addition to being fast mechanically (faster disk rotation speeds and faster head movement) and offering higher data densities, Hitachi Ultrastar disk drives employ read caching, and now a new write caching feature called Hitachi Cache Optimizer. These performance-enhancing techniques make system operations more efficient by helping to speed up data retrieval and transfer.

Disk cache, solid-state memory located on a disk drive, has two functions. First, it bridges data on the disk with data on the interface to the computer. Data cannot travel directly between the disk and the computer because each operates at a different speed. The cache acts as a buffer, matching the speeds of the two. Second, disk cache acts as a holding area for data being written to or retrieved from the disk, improving the response time of the disk, potentially increasing the performance of the entire computing system.

What is read caching?
Read caching is commonly used on most hard disk drives today. Retrieving data from cache takes much less time than from disk--microseconds to read from solid-state memory compared to milliseconds required for the disk read head to move and the disk to rotate to where the data is located. When a read command arrives at the hard drive, the drive first checks to see whether the requested data is in the cache. If the cache does not contain the requested data, the drive retrieves the data from the disk. Drives also may read ahead on the disk for more data than was requested. All that data is saved in the cache. Since many applications process data sequentially, it is likely that the next read request will be for data that follows the last request. Thus, subsequent commands could access that prefetched data from the cache instead of from the disk.

When data can be supplied from the cache, performance can be greatly enhanced. Up to a point, larger caches provide greater performance gains than smaller caches. For example, in server benchmarks, Hitachi has seen significant increase in disk drive cache hit rate with a 8 MB cache compared to a 2 MB cache.

What is write caching?
Like read caching, write caching is a way for the hard disk drive to improve system performance by anticipating what the server will do next. If write caching is enabled on the disk drive, when the server sends a write command to the drive, it immediately tells the computer that the write operation is completed (even though the data has not yet been written to the disk). The data is stored in the write cache and written to the disk later (when the write head has been positioned over the data area and the disk has rotated to the location where the data will be written). By immediately telling the computer the data is written, the computer does not have to wait and can go on with its computing tasks. Eliminating the time the computer waits for writes to complete may improve system performance.

Write caching allows sequential data to be transferred in a continuous flow to the drive, rather than as individual blocks of data separated by disk seek and rotation delays. This is achieved by taking advantage of the ability to write blocks of data sequentially on a disk that is formatted with a 1:1 interleave. As the last byte of data is transferred out of the write buffer onto the disk and the head passes over the next sector of the disk, the first byte of the next block of data is ready to be transferred. This makes it possible to achieve an interruption-free sequential data transfer.

Figure 1: Steps involved in saving a spreadsheet file on a disk with write caching.

STEP	ACTION
1	A user clicks "save" in the spreadsheet program. The file is located on a server disk drive.
2	The server sends information to the hard disk over the SCSI interface. This information consists of several blocks of data that comprise the spreadsheet file.
3	The server disk drive puts the first block of data into its write cache and tells the computer that the write operation is complete. The drive moves the write head to the specified track and records the data after the disk rotates to the specified sector.
4	In the meantime, the computer immediately sends more blocks of information, each time getting an immediate "operation complete" response from the disk drive.
5	After the head and the disk platter are positioned at the specified location, writing begins. Since the blocks are likely to be written to sectors that follow one another, they are written out quickly. Subsequent blocks go into the cache and onto the disk after the platter rotates to the specified location.
6	This process continues without the server having to wait for seek or rotation delays until all the blocks of data that contain the spreadsheet are written to the disk.

Figure 1 shows a simplified example of write caching. Although the example does not show every detail of how servers write to disks, it does convey the overall concept of write caches.

Write caching can significantly improve server performance. It is not unusual to see a 10 percent to 15 percent improvement in performance by enabling write caching. The largest performance gains with this new Hitachi Ultrastar feature are available when the disk drive is doing relatively short-seek random write operations. This is especially true for logical environments where no queueing at all is used. These reordering algorithms allow the drive to execute the disk transfers in an order which minimizes disk access time. Without it, reordering effectiveness is greatly reduced in those environments.

Issues and benefits of write caching
Overall system performance is improved by eliminating the server having to wait for writes to complete. There is a small amount of time where the host software thinks the data is safely written on the disk, yet it is held in volatile cache memory on the disk drive. The host server has no visibility that the information sent to the disk drive was not written to the media since all commands are returned with good status to the system.

If an error or power failure occurs, the host software must verify what is actually written on the disk and take appropriate steps to rewrite the data as part of the restart process. For most computing environments, disk errors and power outages are rare. In addition, many manufacturers are adding battery backup to their computing systems so that even if a power outage or equipment failure occurs, decreasing the chance that cached data would be lost.

Write caching in Hitachi server disk drives
Hitachi began implementing full write caching, called Cache Optimizer, as an option available on Hitachi disk drives in May 1999 with the announment of the Ultrastar 18LZX and is currently available in the Ultrastar 146Z10. The feature can be turned on and off from the server through the SCSI interface. This enables users to gain the performance benefits of write caching while also having the ability to turn it off if the exposure to data loss is a concern.

What's a cache?
A disk cache is solid-state memory on a disk drive that performs two functions. First, it is used to simply match the data transfer speed differences between high-speed computers and slower-speed hard disks. Second, it is used as a holding area (or cache) for data being written to or retrieved from a disk, improving performance. The following are the two kinds of caching used with hard disk drives:

With read caching, the hard disk drive reads more data than was requested by the computer and saves it in the cache. Often, the next read request references data that is adjacent to the last request. Thus, system performance is improved because the disk can provide the data very quickly from the cache.

If write caching is enabled on the disk drive, the disk drive immediately tells the computer that the data was written to the disk, even though it may take some time for the drive to actually seek and the disk to rotate to the desire location. System performance is improved because the computer is free to continue processing rather than waiting for the disk drive to actually write the data on the disk.

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