Книга: Distributed operating systems

5.3.1. New Hardware

5.3.1. New Hardware

Before looking at new hardware, let us look at old hardware with new prices. As memory continues to get cheaper and cheaper, we may see a revolution in the way file servers are organized. Currently, all file servers use magnetic disks for storage. Main memory is often used for server caching, but this is merely an optimization for better performance. It is not essential.

Within a few years, memory may become so cheap that even small organizations can afford to equip all their file servers with gigabytes of physical memory. As a consequence, the file system may permanently reside in memory, and no disks will be needed. Such a step will give a large gain in performance and will greatly simplify file system structure.

Most current file systems organize files as a collection of blocks, either as a tree (e.g., UNIX) or as a linked list (e.g., MS-DOS). With an in-core file system, it may be much simpler to store each file contiguously in memory, rather than breaking it up into blocks. Contiguously stored files are easier to keep track of and can be shipped over the network faster. The reason that contiguous files are not used on disk is that if a file grows, moving it to an area of the disk with more room is too expensive. In contrast, moving a file to another area of memory is feasible.

Main memory file servers introduce a serious problem, however. If the power fails, all the files are lost. Unlike disks, which do not lose information in a power failure, main memory is erased when the electricity is removed. The solution may be to make continuous or at least incremental backups onto videotape. With current technology, it is possible to store about 5 gigabytes on a single 8mm videotape that costs less than 10 dollars. While access time is long, if access is needed only once or twice a year to recover from power failures, this scheme may prove irresistible.

A hardware development that may affect file systems is the optical disk. Originally, these devices had the property that they could be written once (by burning holes in the surface with a laser), but not changed thereafter. They were sometimes referred to as WORM (Write Once Read Many) devices. Some current optical disks use lasers to affect the crystal structure of the disk, but do not damage them, so they can be erased.

Optical disks have three important properties:

1. They are slow.

2. They have huge storage capacities.

3. They have random access.

They are also relatively cheap, although more expensive than videotape. The first two properties are the same as videotape, but the third opens the following possibility. Imagine a file server with an n –gigabyte file system in main memory, and an n –gigabyte optical disk as backup. When a file is created, it is stored in main memory and marked as not yet backed up. All accesses are done using main memory. When the workload is low, files that are not yet backed up are transferred to the optical disk in the background, with byte k in memory going to byte k on the disk. Like the first scheme, what we have here is a main memory file server, but with a more convenient backup device having a one-to-one mapping with the memory.

Another interesting hardware development is very fast fiber optic networks. As we discussed earlier, the reason for doing client caching, with all its inherent complications, is to avoid the slow transfer from the server to the client. But suppose that we could equip the system with a main memory file server and a fast fiber optic network. It might well become feasible to get rid of the client's cache and the server's disk and just operate out of the server's memory, backed up by optical disk. This would certainly simplify the software.

When studying client caching, we saw that a large fraction of the trouble is caused by the fact that if two clients are caching the same file and one of them modifies it, the other does not discover this, which leads to inconsistencies. A little thought will reveal that this situation is highly analogous to memory caches in a multiprocessor. Only there, when one processor modifies a shared word, a hardware signal is sent over the memory bus to the other caches to allow them to invalidate or update that word. With distributed file systems, this is not done.

Why not, actually? The reason is that current network interfaces do not support such signals. Nevertheless, it should be possible to build network interfaces that do. As a very simple example, consider the system of Fig. 5-16 in which each network interface has a bit map, one bit per cached file. To modify a file, a processor sets the corresponding bit in the interface, which is 0 if no processor is currently updating the file. Setting a bit causes the interface to create and send a packet around the ring that checks and sets the corresponding bit in all interfaces. If the packet makes it all the way around without finding any other machines trying to use the file, some other register in the interface is set to 1. Otherwise, it is set to 0. In effect, this mechanism provides a way to globally lock the file on all machines in a few microseconds.

After the lock has been set, the processor updates the file. Each block of the file that is changed is noted (e.g., using bits in the page table). When the update is complete, the processor clears the bit in the bit map, which causes the network interface to locate the file using a table in memory and automatically deposit all the modified blocks in their proper locations on the other machines. When the file has been updated everywhere, the bit in the bit map is cleared on all machines.

Fig. 5-16. A hardware scheme to updating shared files.

Clearly, this is a simple solution that can be improved in many ways, but it shows how a small amount of well-designed hardware can solve problems that are difficult to handle in software. It is likely that future distributed systems will be assisted by specialized hardware of various kinds.

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