What is the compression ratio or algorithm used in VVR replication

[Zahid_Haseeb]

Environment

SFHA/DR = 6.0.2

OS = Win2008R2

[Gaurav_S]

Hi Zahid,

Compression ratio is as such not defined as what exact ratio is being used. I am assuming it would be hard coded.

From Admin guide

Compression feature enables VVR to send data in a compressed form from a

PrimarytoaSecondaryhost.ItreducesnetworkbandwidthconsumptionbyVVR.

This feature is particularly useful in scenarios where there is low availability of

bandwidth or where the bandwidth is shared among several applications.

Purchasing an external compression software or hardware for data transfer can

prove costly. Hence, compression feature in VVR is a cost-effective alternative in such scenarios.

Compression option can be enabled on a per RLINK basis either through the CLI

or GUI. If both sides have compression enabled, the Primary site generates the

compresseddataduringsendingofanyupdates.Atanytime,theusercandisable

or enable the compression feature without stopping replication.

GeneralfunctionalityconstraintsforVVRcompressionareasshown.Datashould

not be send in compressed form in the following cases:

■

If either the Primary or Secondary RLINK does not have compression enabled

■

If the compressed data size is greater than the uncompressed data size

■

If the memory for keeping the compressed data could not be allocated on the

Primary side

G

[Zahid_Haseeb]

Thanks Gaurav for your words.

If the memory for keeping the compressed data could not be allocated on the

Primary side

I never see the memory allocation when configuring the replication (via VEA) between primary and DR site.

=============

So, when the compress replication data reaches at DR site it first uncompress replication data and then it writes to DR site volume. Correct ?

[Gaurav_S]

Yes, that is correct, data is compressed only to save the bandwidth, it will be uncompressed again before getting written to the volumes on DR site.

About, memory, I don't believe you would be able to see any specific memory allocation happening for compression, rather the memory used by VVR itself will be shared.

G

[mikebounds · Accepted solution]

See extract from VVR admin guide:

VVR uses the zlib library for performing compression and decompression. For

more information on the zlib compression library, see http://www.zlib.net

zlib uses the "Deflate" algorithm see http://www.zlib.net/feldspar.html  for more information.

Memory is allocated from the VVR NMCOM memory pool 

Mike

[Wally_Heim · Accepted solution]

Hi Zahid,

The VVR compression ratio is dependent on the data being compressed.  VVR uses the NMCom memory pool on the primary to compress the packet being sent.  It then compares the compressed packet in NMCom memory to uncompressed packet in the RDBack pool memory and sends the one that is smaller or the one in the RDBack pool if they are the same size.

You can use the "rlink -e -i 5 stats <rlink_name>" command on the primary to details on compression that is being done live on the server.

"vxtune" command output also shows some of the tunables that can be done with compression. 

I have adjusted the number of threads with a few customers but not many.  The more threads that you allocate to compression the more load it puts on the servers.  It does not change compression ratios.

If you adjust any tunables alway check your vxmemstat output to see how your memory is doing.  If you are hitting max memory then you should think about increasing memory to NMCom or RDBack pool as needed on the primary and secondary.

Thank you,

Wally