Wafl overview
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Transcript of Wafl overview
Spotlight Series
WAFL Overviewhttp://www.netapp.com/tech_library/3002.html
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er to third parties prohibited without prior written NetApp approval
WAFL – No pre-allocated locations (data and metadata blocks are treated equally). Writes go to nearest available free block.
Writing to nearest available free block reduces disk seeking (the #1 performance challenge when using disks).
WAFL: Write Anywhere File LayoutFilesystem for Improved Productivity
Berkeley Fast File System/Veritas File System/NTFS/etc. – Writes to pre-allocated locations (data vs. metadata)
...
1-2 MB Cylinders
...
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WAFL uses integrated RAID4
RAID4 is similar to better known RAID5:
– RAID5: parity is distributed across all disks in the RAID group
– RAID4: parity is contained in a single disk in the RAID group
Tradeoffs with the single parity disk RAID model:
– PRO: The RAID group can be instantly expanded by adding (pre-formatted) data disks.
– CON: The parity disk is perceived to be the ‘hot spot’ in the RAID group, due to intensive XOR parity calculations on it.
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WAFL eliminates the parity hot spot
WAFL overcomes the ‘classic’ parity-disk hotspot issue, by the use of flexible write allocation policies:
– Writes any filesystem block to any disk location (data and meta data)*
– New data does not overwrite old data
– Allocates disk space for many client-write operations at once in a single new RAID-stripe write (no parity re-calculations)
– Writes to stripes that are near each other
– Writes blocks to disk in any order
* except root inode
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Result: Minimal seeks and no hotspot
file1
file3
file2
Typical File System
Long head seeks
especiallyon
paritydisk
file1
file3file2
WAFL
Shorthead seeksacross
all disks
1 file at a time Multiple files at once
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WAFL Combined with NVRAM
WAFL uses NVRAM “consistency points” (NetApp’s flavor of journalling), thus assuring filesystem integrity and fast reboots.
CP flush to disk occurs once every 10 seconds or when NVRAM reaches half full.
NVRAM placement is at the file system operation level, not at the (more typical) block level. This assures self-consistent CP flushes to disk.
No fsck!
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NVRAM
General-purpose NV-RAM
Disk Driver
SemanticWrite Alloc
NFS or CIFS
TCP/ or UDP/IP
NetAppNV-RAM
File System
Disk Driver
NVRAMSemanticWrite Alloc
TCP/ or UDP/IP
File System
NVRAM safe-storesthe disk blocks
NVRAM safe-stores the FS operation
NVRAM placement is key!
NFS or CIFS
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Seek Example in a SAN environment
Assume 4K disk blocks and 5 msec for one seek+rotate
100MB/sec FC bandwidth x .005sec = .5MB worth of data blocks not sent on the FC channel during that seek
.5MB x 1 block/4KB = 128 blocks not sent
Therefore a 5ms seek for just 1 block equates to a 128 block penalty
Conclusion: one seek every 128 blocks or less ( ~1%) wastes at least half of your FC bandwidth!
(seek 1 block)128 blocks 128 blocks (seek 1 block)
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The protocol overhead issue
• Yes, we have TCP/IP overhead.
• Yes, we have double-buffering overhead.
• Yes, we might well have <obscure performance gotcha>.
• Despite all that, we're able to improve performance, even
with databases (now over 40% of NetApp customer base).
• Clearly, we're doing *something* sufficiently right to
make up for the overhead.
Isn’t NAS slower than local disk?
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The protocol overhead issue
• TCP/IP might seem to be a massive overhead, but passing
packets up and down the stack turn out only to consume
microseconds per request.
(For example: 1Ghz CPU speed == 1 ____second clock cycle.
Keep the timing in perspective with today’s CPU speeds!
• TCP/IP might seem to be a massive overhead, but passing
packets up and down the stack turn out only to consume
microseconds per request.
(For example: 1Ghz CPU speed == 1 nanosecond clock cycle. So 1000 extra
CPU cycles for TCP stack = 1000x1ns = 1 microsecond)
• Eliminating head seeks, which WAFL does better than any
other file system thanks to its full integration with RAID,
saves whole milliseconds, eg, a 1000x savings.
(5ms seek)128 blocks 128 blocks (5ms seek)
TCPoverhead
TCP overhead is small by comparison
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NetApp FilersHigh speed, low latency
SFS97_R1 Performance - NFS v3 TCP http://www.spec.org/cgi-bin/osgresults?conf=sfs97r1
F825F825 F880F880FAS960cFAS960c
• RAID protected• Single file system
• RAID protected• Single file system
FAS960FAS960FAS940FAS940
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Summary
WAFL extracts more ops/sec from a single drive due to minimum seeks.
More ops/sec equates to faster overall performance WAFL’s “anywhere” property makes NetApp’s RAID-4 the performance and scalability winner.
Fastest File System in the world with RAID enabled