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Beyond the File System Designing Large Scale File Storage and Serving Cal Henderson.

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Presentation on theme: "Beyond the File System Designing Large Scale File Storage and Serving Cal Henderson."— Presentation transcript:

1 Beyond the File System Designing Large Scale File Storage and Serving Cal Henderson

2 Web Builder 2.02 Hello!

3 Web Builder 2.03 Big file systems? Too vague! What is a file system? What constitutes big? Some requirements would be nice

4 Web Builder 2.04 Scalable Looking at storage and serving infrastructures 1

5 Web Builder 2.05 Reliable Looking at redundancy, failure rates, on the fly changes 2

6 Web Builder 2.06 Cheap Looking at upfront costs, TCO and lifetimes 3

7 Web Builder 2.07 Four buckets Storage Serving BCP Cost

8 Web Builder 2.08 Storage

9 Web Builder 2.09 The storage stack File system Block protocol RAID Hardware ext, reiserFS, NTFS SCSI, SATA, FC Mirrors, Stripes Disks and stuff File protocol NFS, CIFS, SMB

10 Web Builder Hardware overview The storage scale InternalDASSANNAS LowerHigher

11 Web Builder Internal storage A disk in a computer –SCSI, IDE, SATA 4 disks in 1U is common 8 for half depth boxes

12 Web Builder DAS Direct attached storage Disk shelf, connected by SCSI/SATA HP MSA30 – 14 disks in 3U

13 Web Builder SAN Storage Area Network Dumb disk shelves Clients connect via a fabric Fibre Channel, iSCSI, Infiniband –Low level protocols

14 Web Builder NAS Network Attached Storage Intelligent disk shelf Clients connect via a network NFS, SMB, CIFS –High level protocols

15 Web Builder Of course, its more confusing than that

16 Web Builder Meet the LUN Logical Unit Number A slice of storage space Originally for addressing a single drive: –c1t2d3 –Controller, Target, Disk (Slice) Now means a virtual partition/volume –LVM, Logical Volume Management

17 Web Builder NAS vs SAN With SAN, a single host (initiator) owns a single LUN/volume With NAS, multiple hosts own a single LUN/volume NAS head – NAS access to a SAN

18 Web Builder SAN Advantages Virtualization within a SAN offers some nice features: Real-time LUN replication Transparent backup SAN booting for host replacement

19 Web Builder Some Practical Examples There are a lot of vendors Configurations vary Prices vary wildly Lets look at a couple –Ones I happen to have experience with –Not an endorsement ;)

20 Web Builder NetApp Filers Heads and shelves, up to 500TB in 260U FC SAN with 1 or 2 NAS heads

21 Web Builder Isilon IQ 2U Nodes, 3-96 nodes/cluster, TB FC/InfiniBand SAN with NAS head on each node

22 Web Builder Scaling Vertical vs Horizontal

23 Web Builder Vertical scaling Get a bigger box Bigger disk(s) More disks Limited by current tech – size of each disk and total number in appliance

24 Web Builder Horizontal scaling Buy more boxes Add more servers/appliances Scales forever* *sort of

25 Web Builder Storage scaling approaches Four common models: Huge FS Physical nodes Virtual nodes Chunked space

26 Web Builder Huge FS Create one giant volume with growing space –Suns ZFS –Isilon IQ Expandable on-the-fly? Upper limits –Always limited somewhere

27 Web Builder Huge FS Pluses –Simple from the application side –Logically simple –Low administrative overhead Minuses –All your eggs in one basket –Hard to expand –Has an upper limit

28 Web Builder Physical nodes Application handles distribution to multiple physical nodes –Disks, Boxes, Appliances, whatever One volume per node Each node acts by itself Expandable on-the-fly – add more nodes Scales forever

29 Web Builder Physical Nodes Pluses –Limitless expansion –Easy to expand –Unlikely to all fail at once Minuses –Many mounts to manage –More administration

30 Web Builder Virtual nodes Application handles distribution to multiple virtual volumes, contained on multiple physical nodes Multiple volumes per node Flexible Expandable on-the-fly – add more nodes Scales forever

31 Web Builder Virtual Nodes Pluses –Limitless expansion –Easy to expand –Unlikely to all fail at once –Addressing is logical, not physical –Flexible volume sizing, consolidation Minuses –Many mounts to manage –More administration

32 Web Builder Chunked space Storage layer writes parts of files to different physical nodes A higher-level RAID striping High performance for large files –read multiple parts simultaneously

33 Web Builder Chunked space Pluses –High performance –Limitless size Minuses –Conceptually complex –Can be hard to expand on the fly –Cant manually poke it

34 Web Builder Real Life Case Studies

35 Web Builder GFS – Google File System Developed by … Google Proprietary Everything we know about it is based on talks theyve given Designed to store huge files for fast access

36 Web Builder GFS – Google File System Single Master node holds metadata –SPF – Shadow master allows warm swap Grid of chunkservers –64bit filenames –64 MB file chunks

37 Web Builder GFS – Google File System 1(a)2(a) 1(b) Master

38 Web Builder GFS – Google File System Client reads metadata from master then file parts from multiple chunkservers Designed for big files (>100MB) Master server allocates access leases Replication is automatic and self repairing –Synchronously for atomicity

39 Web Builder GFS – Google File System Reading is fast (parallelizable) –But requires a lease Master server is required for all reads and writes

40 Web Builder MogileFS – OMG Files Developed by Danga / SixApart Open source Designed for scalable web app storage

41 Web Builder MogileFS – OMG Files Single metadata store (MySQL) –MySQL Cluster avoids SPF Multiple tracker nodes locate files Multiple storage nodes store files

42 Web Builder MogileFS – OMG Files Tracker MySQL

43 Web Builder MogileFS – OMG Files Replication of file classes happens transparently Storage nodes are not mirrored – replication is piecemeal Reading and writing go through trackers, but are performed directly upon storage nodes

44 Web Builder Flickr File System Developed by Flickr Proprietary Designed for very large scalable web app storage

45 Web Builder Flickr File System No metadata store –Deal with it yourself Multiple StorageMaster nodes Multiple storage nodes with virtual volumes

46 Web Builder Flickr File System SM

47 Web Builder Flickr File System Metadata stored by app –Just a virtual volume number –App chooses a path Virtual nodes are mirrored –Locally and remotely Reading is done directly from nodes

48 Web Builder Flickr File System StorageMaster nodes only used for write operations Reading and writing can scale separately

49 Web Builder Serving

50 Web Builder Serving files Serving files is easy! ApacheDisk

51 Web Builder Serving files Scaling is harder ApacheDisk ApacheDisk ApacheDisk

52 Web Builder Serving files This doesnt scale well Primary storage is expensive –And takes a lot of space In many systems, we only access a small number of files most of the time

53 Web Builder Caching Insert caches between the storage and serving nodes Cache frequently accessed content to reduce reads on the storage nodes Software (Squid, mod_cache) Hardware (Netcache, Cacheflow)

54 Web Builder Why it works Keep a smaller working set Use faster hardware –Lots of RAM –SCSI –Outer edge of disks (ZCAV) Use more duplicates –Cheaper, since theyre smaller

55 Web Builder Two models Layer 4 –Simple balanced cache –Objects in multiple caches –Good for few objects requested many times Layer 7 –URL balances cache –Objects in a single cache –Good for many objects requested a few times

56 Web Builder Replacement policies LRU – Least recently used GDSF – Greedy dual size frequency LFUDA – Least frequently used with dynamic aging All have advantages and disadvantages Performance varies greatly with each

57 Web Builder Cache Churn How long do objects typically stay in cache? If it gets too short, were doing badly –But it depends on your traffic profile Make the cached object store larger

58 Web Builder Problems Caching has some problems: –Invalidation is hard –Replacement is dumb (even LFUDA) Avoiding caching makes your life (somewhat) easier

59 Web Builder CDN – Content Delivery Network Akamai, Savvis, Mirror Image Internet, etc Caches operated by other people –Already in-place –In lots of places GSLB/DNS balancing

60 Web Builder Edge networks Origin

61 Web Builder Edge networks Origin Cache

62 Web Builder CDN Models Simple model –You push content to them, they serve it Reverse proxy model –You publish content on an origin, they proxy and cache it

63 Web Builder CDN Invalidation You dont control the caches –Just like those awful ISP ones Once something is cached by a CDN, assume it can never change –Nothing can be deleted –Nothing can be modified

64 Web Builder Versioning When you start to cache things, you need to care about versioning –Invalidation & Expiry –Naming & Sync

65 Web Builder Cache Invalidation If you control the caches, invalidation is possible But remember ISP and client caches Remove deleted content explicitly –Avoid users finding old content –Save cache space

66 Web Builder Cache versioning Simple rule of thumb: –If an item is modified, change its name (URL) This can be independent of the file system!

67 Web Builder Virtual versioning Database indicates version 3 of file Web app writes version number into URL Request comes through cache and is cached with the versioned URL mod_rewrite converts versioned URL to path Version 3 Cached: foo_3.jpg foo_3.jpg -> foo.jpg

68 Web Builder Authentication Authentication inline layer –Apache / perlbal Authentication sideline –ICP (CARP/HTCP) Authentication by URL –FlickrFS

69 Web Builder Auth layer Authenticator sits between client and storage Typically built into the cache software Cache Authenticator Origin

70 Web Builder Auth sideline Authenticator sits beside the cache Lightweight protocol used for authenticator Cache Authenticator Origin

71 Web Builder Auth by URL Someone else performs authentication and gives URLs to client (typically the web app) URLs hold the keys for accessing files CacheOriginWeb Server

72 Web Builder BCP

73 Web Builder Business Continuity Planning How can I deal with the unexpected? –The core of BCP Redundancy Replication

74 Web Builder Reality On a long enough timescale, anything that can fail, will fail Of course, everything can fail True reliability comes only through redundancy

75 Web Builder Reality Define your own SLAs How long can you afford to be down? How manual is the recovery process? How far can you roll back? How many node x boxes can fail at once?

76 Web Builder Failure scenarios Disk failure Storage array failure Storage head failure Fabric failure Metadata node failure Power outage Routing outage

77 Web Builder Reliable by design RAID avoids disk failures, but not head or fabric failures Duplicated nodes avoid host and fabric failures, but not routing or power failures Dual-colo avoids routing and power failures, but my need duplication too

78 Web Builder Tend to all points in the stack Going dual-colo: great Taking a whole colo offline because of a single failed disk: bad We need a combination of these

79 Web Builder Recovery times BCP is not just about continuing when things fail How can we restore after they come back? Host and colo level syncing –replication queuing Host and colo level rebuilding

80 Web Builder Reliable Reads & Writes Reliable reads are easy –2 or more copies of files Reliable writes are harder –Write 2 copies at once –But what do we do when we cant write to one?

81 Web Builder Dual writes Queue up data to be written –Where? –Needs itself to be reliable Queue up journal of changes –And then read data from the disk whose write succeeded Duplicate whole volume after failure –Slow!

82 Web Builder Cost

83 Web Builder Judging cost Per GB? Per GB upfront and per year Not as simple as youd hope –How about an example

84 Web Builder Hardware costs Cost of hardware Usable GB Single Cost

85 Web Builder Power costs Cost of power per year Usable GB Recurring Cost

86 Web Builder Power costs Power installation cost Usable GB Single Cost

87 Web Builder Space costs Cost per U Usable GB [ ] Us needed (inc network) x Recurring Cost

88 Web Builder Network costs Cost of network gear Usable GB Single Cost

89 Web Builder Misc costs Support contracts + spare disks Usable GB + bus adaptors + cables [ ] Single & Recurring Costs

90 Web Builder Human costs Admin cost per node Node count x Recurring Cost Usable GB [ ]

91 Web Builder TCO Total cost of ownership in two parts –Upfront –Ongoing Architecture plays a huge part in costing –Dont get tied to hardware –Allow heterogeneity –Move with the market

92 (fin)

93 Web Builder Photo credits / / / / / / / /

94 Web Builder You can find these slides online:

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