Simulator

This section introduces raid.py, a simple RAID simulator you can use to shore up your knowledge of how RAID systems work.

It has a number of options, as we see below:

Usage: raid.py [options]

Options:

  -h, --help            show this help message and exit
  -s SEED, --seed=SEED  the random seed
  -D NUMDISKS, --numDisks=NUMDISKS
                        number of disks in RAID
  -C CHUNKSIZE, --chunkSize=CHUNKSIZE
                        chunk size of the RAID
  -n NUMREQUESTS, --numRequests=NUMREQUESTS
                        number of requests to simulate
  -S SIZE, --reqSize=SIZE
                        size of requests
  -W WORKLOAD, --workload=WORKLOAD
                        either "rand" or "seq" workloads
  -w WRITEFRAC, --writeFrac=WRITEFRAC
                        write fraction (100->all writes, 0->all reads)
  -R RANGE, --randRange=RANGE
                        range of requests (when using "rand" workload)
  -L LEVEL, --level=LEVEL
                        RAID level (0, 1, 4, 5)
  -5 RAID5TYPE, --raid5=RAID5TYPE
                        RAID-5 left-symmetric "LS" or left-asym "LA"
  -r, --reverse         instead of showing logical ops, show physical
  -t, --timing          use timing mode, instead of mapping mode
  -c, --compute         compute answers for me

In its basic mode, you can use it to understand how the different RAID levels map logical blocks to underlying disks and offsets. For example, let’s say we wish to see how a simple striping RAID (RAID-0) with four disks does this mapping.

prompt> ./raid.py -n 5 -L 0 -R 20 
...
LOGICAL READ from addr:16 size:4096
  Physical reads/writes?

LOGICAL READ from addr:8 size:4096
  Physical reads/writes?

LOGICAL READ from addr:10 size:4096
  Physical reads/writes?

LOGICAL READ from addr:15 size:4096
  Physical reads/writes?

LOGICAL READ from addr:9 size:4096
  Physical reads/writes?

In this example, we simulate five requests (-n 5), specifying RAID level zero (-L 0), and restrict the range of random requests to just the first twenty blocks of the RAID (-R 20). The result is a series of random reads to the first twenty blocks of the RAID; the simulator then asks you to guess which underlying disks/offsets were accessed to service the request, for each logical read.

In this case, calculating the answers is easy: in RAID-0, recall that the underlying disk and offset that services a request is calculated via modulo arithmetic: ...