Storage Area Networks For DummiesÂ®

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350 Par t V: Understanding the Cool Stuff The trade-off is the size of the index table and the number of entries required. As you see in Figure 13-11, fewer hashes are required when you use larger blocks; therefore, the index table will be smaller and faster. On the other hand, your de-dupe ratio is not as efficient when you use larger blocks. Always ask your vendor whether its solution is tunable based on your policies, so that you can determine whether you want to use more disk space and fewer hash entries to get a faster solution, or use less disk space and more hash entries to get a solution that is a bit slower but generates much better de-dupe ratios. Creating a policy is especially useful for solutions that work out of band; because performance is less of an issue (the backup job is already complete) you can use more time but get a better de-dupe ratio by using smaller chunks. Hash Updates to Index Table Using 4KB Chunks = 3 Entries Hash Entries in Index Table Using 512-byte Chunks = 24 Entries Figure 13-11: Hash index table entries using different data chunk sizes. 74476bea33bce9985dbf96 23476bea872cd9985bcad3 9757fa23472cd9985b76bc Hashes stored in the index 9757fa23472cd9987c98bc fb5761298ca6597367234a 5e7623ac6981bd4d32a972 bce6df3987a234a78923c6 7664baf4987a675cbff3269 75bcd4eaf39872fccb46aa 897cde458712accff76499 7664baf4987a675cbff3269 75bcd4eaf39872fccb46aa 897cde458712accff76499 75bcd4eaf39872fccb46aa 9757fa23472cd9987c98bc fb5761298ca6597367234a 5e7623ac6981bd4d32a972 5e7623ac6981bd4d32a972 7664baf4987a675cbff3269 75bcd4eaf39872fccb46aa 897cde458712accff76499 7664baf4987a675cbff3269 7779b2c3598cadef319876 7779b2c3598cadef319876 7779b2c3598cadef319876 9757fa23472cd9987c98bc 7779b2c3598cadef319876 7779b2c3598cadef319876 7779b2c3598cadef319876 Hashes stored in the index What about hash collisions? A hash collision isn’t a traffic accident, but it could have the same ramifications for your business. A hash collision happens when the resulting hashes for two unique chunks of data are the same. Data de-duplication finds redundancy by hashing blocks of data and comparing the hashes to determine whether two data blocks are the same or unique. In this example, two similar inputs
Chapter 13: Using Data De-Duplication to Lighten the Load 351 would have the same output, and the compare operation would determine that because the two inputs are the same, one can be deleted. The problem with a hash collision is that if the two inputs are different but the output is the same (hash collision), and one of the inputs is deleted, you could lose data. The following examples involve the MD5 hashing algorithm, which uses a 128-bit hash made from 32 hexadecimal digits. The resulting hash for two matching blocks of data is the same. To keep things simple, the examples use letters of the alphabet. Example 1: Hash match, data is redundant In this example, you input two letters (two As) into the algorithm: Hash for Letter A = A4BEC893BEE67418975BEAAC53298721 Hash for Letter A = A4BEC893BEE67418975BEAAC53298721 The resulting hashes match, so the letters are duplicates, and one can be eliminated. Everyone’s happy. Example 2: Hash match, data is unique (hash collision) In this example, you input two letters (A and B) into the algorithm, and the resulting hashes collide: Hash for Letter A = A4BEC893BEE67418975BEAAC53298721 Hash for Letter B = A4BEC893BEE67418975BEAAC53298721 Because the hashes match, the algorithm thinks you can eliminate one of the letters, but that would result in the loss of data. The algorithm is in error. The probability of a hash collision actually happening using 128-bit hash is virtually nil, so you should not worry. As an example, compare the probability of a hash collision with other ludicrously unlikely events: ✓ The chance of a killer asteroid striking our planet at any given minute and wiping out most life on Earth is roughly 1 in 10 13 , so it’s more probable than a hash collision. ✓ Using a combined hash of MD5 and SHA-1, the possibility of a hash collision is about as likely as purchasing eight consecutive winning lottery tickets. So while you’re counting your money while you wait for the asteroid to hit, you can be reassured in the fact that at least your data is safe. In other words, chill out about hash collisions.
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Storage Area Networks For Dummies
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Author’s Acknowledgments Christop
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Contents at a Glance Introduction .
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In this part . . . The computer ind
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8 Par t I: SAN 101 In a nutshell, y
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10 Par t I: SAN 101 Disk drives in
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12 Par t I: SAN 101 Finding Out Whe
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14 Par t I: SAN 101 ✓ Servers tha
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16 Par t I: SAN 101 ✓ Gateway: A
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18 Par t I: SAN 101 on the other ha
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20 Par t I: SAN 101 All Fibre Chann
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22 Par t I: SAN 101 ✓ Any backup
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24 Par t I: SAN 101 Host layer Fabr
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26 Par t I: SAN 101 Newer drive-int
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28 Par t I: SAN 101 ✓ Long-wave:
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30 Par t I: SAN 101 Large-scale, co
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32 Par t I: SAN 101 Hubs typically
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34 Par t I: SAN 101 You could conne
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36 Par t I: SAN 101 Using modular s
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38 Par t I: SAN 101 Data routers A
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40 Par t I: SAN 101 Cables Cables a
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42 Par t I: SAN 101 Core (9-micron
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44 Par t I: SAN 101 Basic SAN port
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46 Par t I: SAN 101 Table 2-2 (cont
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48 Par t I: SAN 101 a Fibre Channel
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50 Par t I: SAN 101 The RAID type t
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52 Par t I: SAN 101 ✓ RAID 2: RAI
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54 Par t I: SAN 101 RAID 0+1 set: D
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56 Par t I: SAN 101 downtime is a p
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58 Par t I: SAN 101 These arrays ar
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60 Par t I: SAN 101 Most SAN storag
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66 Par t I: SAN 101 Bandwidth Movin
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70 Par t I: SAN 101 The Switched Fa
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72 Par t I: SAN 101 Again, think of
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74 Par t I: SAN 101 The switches’
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76 Par t I: SAN 101 So what can you
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78 Par t I: SAN 101 For example, if
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82 Par t I: SAN 101 You need to rep
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84 Par t I: SAN 101 Little things c
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86 Par t I: SAN 101 If you’re pur
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88 Par t I: SAN 101 Dual I/O paths
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90 Par t I: SAN 101 All switch vend
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In this part . . . If this were a b
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94 Par t II: Designing and Building
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