1 Firebird and RAID Choosing the right RAID configuration for Firebird. Paul Reeves IBPhoenix mail:
2 Introduction Disc drives have become so cheap that implementing RAID for a firebird server is now very affordable.
3 Intended Audience It is hard enough to sell clients your application, never mind get them to invest in a suitable server. Your clients don't have much of an in-house IT dept. Your Sys Admins don't see why you need a dedicated RAID array for your departmental server. Unintended Audience If you are working for a company that can afford a million dollar SAN this talk may not be much use to you.
4 The focus of this talk Primarily looking at the day to day performance issues that underly RAID Data recovery is not really considered. However data recovery is heavily impacted by choice of RAID.
5 What is RAID? Redundant Array of Inexpensive Discs Redundant if one disc fails the system continues. Array Discs grouped together bring better performance. Inexpensive Many cheap discs combined can work better than more expensive discs.
6 RAID is NOT An alternative backup strategy RAID exists to overcome disc failure, not data corruption. Data corruption is copied to all discs so always make sure you make backups.
7 Redundancy has a price All RAID implementations require writing to more than one disc. Database updates can actually become slower (in extreme cases).
8 Forget about the fancy RAID names There are just two basic types of RAID configuration Mirrored Parity And there is two types of No RAID at all JBOD Disc striping (concatenating) RAID 0
9 JBOD Just a Bunch Of Discs Where would we be without acronyms?
10 RAID 0 Good for creating very large discs A disaster waiting to happen. Not much use for database storage. Becomes very useful when combined with other RAID levels.
11 Mirrored RAID Maintain identical data on two or more discs Each disc in the array requires a write. Usually implemented as RAID 1 or RAID 10
12 Parity RAID Writes data blocks on every N discs -1 plus parity block(s). Distribution of data and parity blocks is evenly distributed across all discs. All discs in array must be written to. Calculating parity costs read I/O. Usually implemented as RAID 5, RAID 50, RAID 6 or RAID 60.
13 Combining RAID levels. Two or more arrays are concatenated to make a larger array.
14 Choosing the correct RAID level Calculating Hard Disc performance IOPS Input / Output Operations Per Second For hard drives we first need to calculate average latency: Avg Latency = (60 / RPMs / 2) * 1000 We then take the average seek time for the drive and derive the IOPS: IOPS = 1000 / (avg latency + avg seek)
15 A rough guide to IOPS for different disc speeds Manufacturers don't always provide full specifications but we can make a good guess. RPM Avg Latency Avg Read Seek RIOPS Avg Write Seek WIOPS 5, , , ,
16 What does IOPS really mean? IOPS is a theoretical value. As such it has no relation to actual data throughput. IOPS indicates the maximum number of times per second that a drive could randomly read or write to a disc.
17 Random and Sequential Access - not what they seem Sequential access is almost non-existent on a server if more than one process is accessing the disc Random is rarely random usually several blocks can be written in a single I/O.
18 The Write Penalty RAID Level Min. No. Disks Write Penalty Comment JBOD / RAID One disc. One write. RAID Write penalty is directly related to the number of disks in the mirror. For three disks the penalty is 3. RAID Every write requires a read of the data block in the stripe, a read of the existing parity block then the actual write of the updated data block and parity block. RAID As for RAID 5 except extra parity block requires an additional read and write
19 Calculating RAID performance There is a simple formula: ( ( DISK_WIOPS * NO_DISCS * %WRITES ) / WRITE_PENALTY ) + ( DISK_RIOPS * NO_DISCS * %READS ) = Theoretical maximum random IOPS for a given array.
20 Using Two or Three Discs in a RAID
21 Four Disc RAID configurations
22 Six Disc RAID configurations
23 Eight Disc RAID configurations
24 Can slower discs be better value than faster discs?
25 Summary of Theory Adding discs increases available IOPS. The Write Penalty is real. Write intensive applications always benefit from Mirrored RAID. For a given number of discs Mirrored RAID will always outperform Parity RAID in Random I/O unless the database is read only.
26 What about reality? So much for theory.
27 First, you need a RAID Controller Firmware based RAID controllers Software based RAID controllers Hardware based RAID controllers
28 Firmware RAID AKA Fake RAID or Bios RAID. Usually built into Motherboard or on cheap disc controller cards. Not easily portable in the event of failure. Requires CPU and RAM from host. It brings the benefit of configuration in the bios. By extension, this allows the O/S to boot from the RAID. But it also renders remote recovery a problem.
29 Software RAID Part of the O/S No special hardware required Requires CPU and RAM from Host. No Vendor Lock-in. Portable if Host fails just pop the discs into another computer. Linux implementation rich in functionality aims to provide top class RAID support. No BBU
30 Hardware RAID CPU independent Built-in cache Battery backup of cache Ease of configuration Multi-platform client GUI (HP, IBM?) Disc monitoring Hot spares Hot swapping Vendor lock-in.
31 A word about Stripe/Strip size What is it? Does it affect RAID choice? Does it affect FB page size?
32 RAID, Strip and Page Size
33 RAID Performance in the real world Case study HP Smart Array 410 Case study s/w RAID on Linux
34 INSERTS comparison HW RAID
35 INSERTS comparison SW RAID
36 UPDATES comparison HW RAID
37 UPDATES comparison SW RAID
38 SELECTS comparison HW RAID
39 SELECTS comparison SW RAID Results similar to HW RAID
40 And what about SSD in all this? SSD raises the bar IOPS do increase massively. Wear Levelling, TRIM, Garbage Collection and Write Amplification pose real problems for database use especially for MLC based flash drives. RAID and TRIM don't (yet) work together. Not all SSDs are created equal check benchmarks from a reliable h/w test site. Don't believe the manufacturers specs. Do your own real world tests. Smaller drives seem to have poorer performance! Price / Capacity ratio is still a hindrance to uptake. SS Drives can still fail and when they do, failure is total.
41 Conclusion We've compared parity and mirrored RAID at the fundamental, theoretical level. We've also looked at some real world examples of RAID. Although parity RAID can be tweaked it cannot out perform a mirrored RAID implementation of the same spec when deploying a database server.
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