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# Operating system questions : RAID

Please refer to attached document below.
1.Suppose you have two RAID arrays, one implementing RAID 3, the other RAID 5. Each has 9 disk drives in its array. If the RAID 5 array can read a 1 Kbyte of data in 16 ms, how long would you expect the RAID 3 array to take to read 1 Kbyte of data? Why?
2. Suppose now your two RAIDs are a RAID 5 and a RAID 6. Each of these RAIDs has a read speed of 48 MB/sec. In the event of a disk failure in either of these RAIDs, read performance is degraded by 12% as data from the failed disk must now be re-constructed from the RAID’s redundant data. For each of the cases below, assume the 875 Kbytes of data to be read is in physically sequential sectors on the disk, so that no additional seeks or delays occur once reading begins.
(a)If both RAIDs perform normally (i.e., all drives working normally), how long does it take for the RAID 5 to read this data? for the RAID 6?
(b) Suppose one disk has failed in each of the arrays. How long does it take for the RAID 5 to read this data? for the RAID 6?
(c) Suppose a total of two disks have failed in each of the arrays. How long does it take for the RAID 5 to read this data? for the RAID 6?

## Operating system questions : RAID ect….

3. Suppose you have a disk with 512-byte sectors, 96 sectors/track, 110 tracks per surface and 8 surfaces. How many sectors will you need to store a file that has 300,000 120-byte records? Note that a record cannot span two sectors.
4. Three years ago your company bought a new RAID 5 array. All the disks in the RAID were brand new, all manufactured within a month of each other. Today one disk has failed in this RAID. How many disks do you replace? Why?
5. The file system in an OS needs to keep track of disk blocks that are in use (i.e., part of a file) and those that are not in use (i.e., not yet used in a file, or formerly part of a file now deleted). One way to do this is with a bitmap in which each disk block is represented by 1 bit indicating the block’s status (free or busy). For a filesystem with 1,464,145,922 512-byte blocks, how many 512-byte blocks would be needed to hold the bitmap?

### Operating system questions : RAID ect….

6. Explain why it’s difficult to support direct access to files with variable length records. Suggest a method for handling this type of file if direct access is required.
7. If you have a system with multiple disk drives, is it better to keep all the paging on one disk (e.g., the disk holding the OS), or spread the paging over multiple drives? Briefly explain your answer.
8. You are accustomed to running a defragmentation utility on your Windows file systems to improve performance (by reducing fragmentation). Would you expect the same kind of improvement from ‘defragging’ a file system on a solid-state drive? Why or why not?
9. Explain why the inode for a file that has been opened by a program does not contain the read/write pointer (indicating where, in the file, the program is working).
10. For a *nix system with disk statistics as shown below:
How many more 25 Mbyte files can be added before there’s an error ”Write failed, device full” to sdb1? sdc1?

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