Hi, Takenori:
Thanks for your quick reply. Your explain is clear for me understanding what
compaction mean, and I also can understand now same row key will exist in multi
SSTable file.
But beyond that, I want to know what happen if one row data is too large to put
in one SSTable file. In your example, the same row exist in multi SSTable files
as it is keeping changing and flushing into the disk at runtime. That's fine,
in this case, in every SSTable file of the 4, there is no single file contains
whole data of that row, but each one does contain full picture of individual
unit ( I don't know what I should call this unit, but it will be larger than
one column, right?). Just in your example, there is no way in any time, we
could have SSTable files like following, right:
- Color-1-Data.db: [{Lavender: {hex: #E6E6FA}}, {Blue: {hex: #0000}}]-
Color-1-Data_1.db: [{Blue: {hex:FF}}]
- Color-2-Data.db: [{Green: {hex: #008000}}, {Blue: {hex2: #2c86ff}}]
- Color-3-Data.db: [{Aqua: {hex: #00FFFF}}, {Green: {hex2: #32CD32}}, {Blue:
{}}]
- Color-4-Data.db: [{Magenta: {hex: #FF00FF}}, {Gold: {hex: #FFD700}}]
I don't see any reason Cassandra will ever do that, but just want to confirm,
as your 'no' answer to my 2 question is confusion.
Another question from my originally email, even though I may get the answer
already from your example, but just want to confirm it.Just use your example,
let's say after the first 2 steps:
- Color-1-Data.db: [{Lavender: {hex: #E6E6FA}}, {Blue: {hex: #0000FF}}]
- Color-2-Data.db: [{Green: {hex: #008000}}, {Blue: {hex2: #2c86ff}}]There is a
incremental backup. After that, there is following changes coming:
- Add a column of (key, column, column_value = Green, hex2, #32CD32)
- Add a row of (key, column, column_value = Aqua, hex, #00FFFF)
- Delete a row of (key = Blue)---- memtable is flushed => Color-3-Data.db
----Another incremental backup right now.
Now in this case, my assumption is only Color-3-Data.db will be in this backup,
right? Even though Color-1-Data.db and Color-2-Data.db contains the data of the
same row key as Color-3-Data.db, but from a incremental backup point of view,
only Color-3-Data.db will be stored.
The reason I asked those question is that I am thinking to use MapReduce jobs
to parse the incremental backup files, and rebuild the snapshot in Hadoop side.
Of course, the column families I am doing is pure Fact data. So there is
delete/update in Cassandra for these kind of data, just appending. But it is
still important for me to understand the SSTable file's content.
Thanks
Yong
Date: Tue, 17 Sep 2013 11:12:01 +0900
From: ts...@cloudian.com
To: user@cassandra.apache.org
Subject: Re: questions related to the SSTable file
Hi,
> 1) I will expect same row key could show up in both
sstable2json output, as this one row exists in both SSTable files,
right?
Yes.
> 2) If so, what is the boundary? Will Cassandra guarantee the
column level as the boundary? What I mean is that for one column's
data, it will be guaranteed to be either in the first file, or 2nd
file, right? There is no chance that Cassandra will cut the data of
one column into 2 part, and one part stored in first SSTable file,
and the other part stored in second SSTable file. Is my
understanding correct?
No.
> 3) If what we are talking about are only the SSTable files in
snapshot, incremental backup SSTable files, exclude the runtime
SSTable files, will anything change? For snapshot or incremental
backup SSTable files, first can one row data still may exist in more
than one SSTable file? And any boundary change in this case?
> 4) If I want to use incremental backup SSTable files as
the way to catch data being changed, is it a good way to do what I
try to archive? In this case, what happen in the following
example:
I don't fully understand, but snapshot will do. It will create
hard links to all the SSTable files present at snapshot.
Let me explain how SSTable and compaction works.
Suppose we have 4 files being compacted(the last one has bee just
flushed, then which triggered compaction). Note that file names
are simplified.
- Color-1-Data.db: [{Lavender: {hex: #E6E6FA}}, {Blue: {hex:
#0000FF}}]
- Color-2-Data.db: [{Green: {hex: #008000}}, {Blue: {hex2:
#2c86ff}}]
- Color-3-Data.db: [{Aqua: {hex: #00FFFF}}, {Green: {hex2:
#32CD32}}, {Blue: {}}]
- Color-4-Data.db: [{Magenta: {hex: #FF00FF}}, {Gold: {hex:
#FFD700}}]
They are created by the following operations.
- Add a row of (key, column, column_value = Blue, hex, #0000FF)
- Add a row of (key, column, column_value = Lavender, hex,
#E6E6FA)
---- memtable is flushed => Color-1-Data.db ----
- Add a row of (key, column, column_value = Green, hex, #008000)
- Add a column of (key, column, column_value = Blue, hex2,
#2c86ff)
---- memtable is flushed => Color-2-Data.db ----
- Add a column of (key, column, column_value = Green, hex2,
#32CD32)
- Add a row of (key, column, column_value = Aqua, hex, #00FFFF)
- Delete a row of (key = Blue)
---- memtable is flushed => Color-3-Data.db ----
- Add a row of (key, column, column_value = Magenta, hex, #FF00FF)
- Add a row of (key, column, column_value = Gold, hex, #FFD700)
---- memtable is flushed => Color-4-Data.db ----
Then, a compaction will merge all those fragments together into
the latest ones as follows.
- Color-5-Data.db: [{Lavender: {hex: #E6E6FA}, {Aqua: {hex:
#00FFFF}, {Green: {hex: #008000, hex2: #32CD32}}, {Magenta: {hex:
#FF00FF}}, {Gold: {hex: #FFD700}}]
* assuming RandomPartitioner is used
Hope they would help.
- Takenori
(2013/09/17 10:51), java8964 java8964
wrote:
Hi, I have some questions related to the SSTable in
the Cassandra, as I am doing a project to use it and hope
someone in this list can share some thoughts.
My understand is the SSTable is per column family. But each
column family could have multi SSTable files. During the
runtime, one row COULD split into more than one SSTable file,
even this is not good for performance, but it does happen, and
Cassandra will try to merge and store one row data into one
SSTable file during compassion.
The question is when one row is split in multi SSTable
files, what is the boundary? Or let me ask this way, if one
row exists in 2 SSTable files, if I run sstable2json tool to
run on both SSTable files individually:
1) I will expect same row key could show up in both
sstable2json output, as this one row exists in both SSTable
files, right?
2) If so, what is the boundary? Will Cassandra guarantee
the column level as the boundary? What I mean is that for one
column's data, it will be guaranteed to be either in the first
file, or 2nd file, right? There is no chance that Cassandra
will cut the data of one column into 2 part, and one part
stored in first SSTable file, and the other part stored in
second SSTable file. Is my understanding correct?
3) If what we are talking about are only the SSTable files
in snapshot, incremental backup SSTable files, exclude the
runtime SSTable files, will anything change? For snapshot or
incremental backup SSTable files, first can one row data still
may exist in more than one SSTable file? And any boundary
change in this case?
4) If I want to use incremental backup SSTable files as the
way to catch data being changed, is it a good way to do what I
try to archive? In this case, what happen in the following
example:
For column family A:
at Time 0, one row key (key1) has some data. It is being
stored and back up in SSTable file 1.
at Time 1, if any column for key1 has any change (a new
column insert, a column updated/deleted, or even whole row
being deleted), I will expect this whole row exists in the any
incremental backup SSTable files after time 1, right?
What happen if the above row just happen to store in more
than one SSTable file?
at Time 0, one row key (key1) has some data, and it just is
stored in SSTable file1 and file2, and being backup.
at Time 1, if one column is added in row key1, and the
change in fact will happen in SSTable file2 only in this case,
and if we do a incremental backup after that, what SSTable
files should I expect in this backup? Both SSTable files? Or
Just SSTable file 2?
I was thinking incremental backup SSTable files are good
candidate for catching data being changed, but as one row data
could exist in multi SSTable file makes thing complex now. Did
anyone have any experience to use SSTable file in this way?
What are the lessons?
Thanks
Yong
