The question may looks duplicate. But i am not getting the answer which i am looking.
The problem is, in unix, one of the 4GL binary is fetching data from the table using cursor and writing the data in .txt file.
The table contains around 50 Million records.
The binary took lot of time and not completing. the .txt file is also 0 byte.
I want to know the possibilities why the records are not written in the .txt file.
Note: There is enough disk space available.
Also, for 30 Million records, i can get the data in the .txt file as i expected.
The information you provide is insufficient to tell for sure why the file is not written.
In UNIX, a text file is just like any another file - a collection of bytes. No specific limit (or structure) is enforced on "row size" or "row count," although obviously, some programs might have certain limits on maximum supported line sizes and such (depending on their implementation).
When a program starts writing data to a file (i.e. once the internal buffer is flushed for the first time) the file will no longer be zero size, so clearly your binqary is doing something else all that time (unless it wipes out the file as part of the cleanup).
Try running your executable via strace to see the file I/O activity - that would give some clues as to what is going on.
Try closing the writer if you are using one to write to the file. It achieves the dual purpose of closing the resource along with flushing the remaining contents of the buffer.
CPU calculated output needs to be flushed if you are using any mechanism of buffered writer. I have encountered such situations a few times and in almost all cases, the issue was that of flushing the output.
In java specifically, usually the best practice of writing data involves buffers. So when the buffer limit is reached, it gets written to the file but doesn't get written to the file when the end of buffer has not been reached yet. This happens when program closes without flushing the buffered writer.
So, in your case, if the processing time that it takes is reasonable and still the output is not on the file, it may mean that the output has been calculated and put on the RAM but could not be written to the file (which represents the disk) due to the output not being flushed.
You can also consider the answers to this question.
Related
Can the ext4 filesystem detect data corruption of file contents? If yes, is it enabled by default and how can I check for corrupted data?
I have read that ext4 maintains checksums for file metadata and its journal, but I was unable to find any information on checksums for the actual file contents.
For clarity: I want to know if a file has changed since the last write operation.
No, ext4 doesn't and can't detect file content corruption.
Well known file systems implementing silent data corruption detection and therefore able to correct it when enough redundancy is available are ZFS and btrfs.
They do it by computing and storing a CRC for every data block written and by checking the CRC or each data block read. Should the CRC doesn't match the data, the latter is not provided to the caller and either RAID allows for an alternate block to be used instead, or an I/O error is reported.
The reading process will never receive corrupted data, either it is correct or the read fails.
"Can the ext4 filesystem detect data corruption of file contents?"
Not in the sense you are expecting. It performs journaling, creating a boolean {before vs after} copy to ensure io completion.
A CRC / checksum is a test for modification from a known state and although the CRC or checksum may not compare to the original, that does not imply that the file is then "corrupt" (aka invalid) - - it only says it has been changed. Strictly speaking, one form of "corruption" would be to alter the 'magic number' at the beginning of a file, like changing %PDF to %xYz - - that would make the content unusable to any program.
"... to know if a file has changed since the last write operation".
Systems that track mtime() will do so uniformly, so every write will modify mtime() making your request impossible.
The only way mtime() would not reflect last write io would be media degredation.
I am writing continuously into a db file which has PRAGMA journal_mode=WAL, PRAGMA journal_size_limit=0. My C++ program has two threads, one reader(queries at 15 sec intervals) and one writer(inserts at 5 sec intervals).
Every 3 min I am pausing insertion to run a sqlite3_wal_checkpoint_v2() from the writer thread with the mode parameter as SQLITE_CHECKPOINT_RESTART. To ensure that no active read operations are going on at this point, I set a flag that checkpointing is about to take place and wait for reader to complete (the connection is still open) before running checkpoint. After checkpoint completion I again indicate to readers it is okay to resume querying.
sqlite3_wal_checkpoint_v2() returns SQLITE_OK, and pnLog and Ckpt as equal(around 4000), indicating complete wal file has been synced with main db file. So next write should start from beginning according to documentation. However, this does not seem to be happening as the subsequent writes cause the WAL file to grow indefinitely, eventually up to some GBs.
I did some searching and found that that readers can cause checkpoint failure due to open transactions. However, the only reader I'm using is ending its transaction before the checkpoint starts. What else could be preventing the WAL file from not growing?
This is far too late as an answer, but may be useful to other people.
According to the SQLite documentation, your expectations should be correct, but if you read this SO post, problems arise also in case of non-finalized statements. Therefore, if you just sqlite3_reset() your statement, there are chances anyway that the db may look busy or locked for a checkpoint. Note that this may happen also with higher levels of SQLITE_CHECKPOINT_values.
Also, the SQLITE_CHECKPOINT_TRUNCATE value, if checkout is successfully operated, will truncate the -wal file to zero length. That may help you check that all pages have been inserted in the db.
Another discussion in which -wal files grow larger and larger due to unfinalized statements is this.
I am pretty much new in Mongodb now what I want to do is to insert a pdf file of 3MB using JAVA driver and want change the chunk size from 256 to 1mb and then want to retrieve the second chunk say 2nd page of the pdf document.
How can I do so.
Thankyou.
Generally, once a document has been written into GridFS you will need to re-write it (delete and save again) to modify the chunk size.
Since GridFS does not know anything about the format of the data in the file it can not help you get to the "2nd page". The InputStream implementation that is returned from GridFSDBFile does avoid reading blocks when you use the skip(long) method. If you know that the "2nd page" is N bytes into the file then you can skip that many bytes in the stream and start reading.
HTH, Rob
P.S. Remember that skip(long) returns the number of bytes actually skipped. You should not assume that skip(12) always skips 12 bytes.
P.P.S Starting to read from the middle of a PDF and making sense of what is there is going to be hard unless you have preserved state from the previous page(s).
I need to write a client/server app stored on a network file system. I am quite aware that this is a no-no, but was wondering if I could sacrifice performance (Hermes: "And this time I mean really slash.") to prevent data corruption.
I'm thinking something along the lines of:
Create a separate file in the system everytime a write is called (I'm willing do it for every connection if necessary)
Store the file name as the current millisecond timestamp
Check to see if the file with that time or earlier exists
If the same one exists wait a random time between 0 to 10 ms, and try again.
While file is the earliest timestamp, do work, delete file lock, otherwise wait 10ms and try again.
If a file persists for more than a minute, log as an error, stop until it is determined that the data is not corrupted by a person.
The problem I see is trying to maintain the previous state if something locks up. Or choosing to ignore it, if the state change was actually successful.
Is there a better way of doing this, that doesn't involve not doing it this way? Or has anyone written one of these with a lot less problems than the Sqlite FAQ warns about? Will these mitigations even factor in to preventing data corruption?
A couple of notes:
This must exist on an NSF, the why is not important because it is not my decision to make (it doesn't look like I was clear enough on that point).
The number of readers/writers on the system will be between 5 and 10 all reading and writing at the same time, but rarely on the same record.
There will only be clients and a shared memory space, there is no way to put a server on there, or use a server based RDMS, if there was, obviously I would do it in a New York minute.
The amount of data will initially start off at about 70 MB (plain text, uncompressed), it will grown continuous from there at a reasonable, but not tremendous rate.
I will accept an answer of "No, you can't gain reasonably guaranteed concurrency on an NFS by sacrificing performance" if it contains a detailed and reasonable explanation of why.
Yes, there is a better way. Don't use NFS to do this.
If you are willing to create a new file every time something changes, I expect that you have a small amount of data and/or very infrequent changes. If the data is small, why use SQLite at all? Why not just have files with node names and timestamps?
I think it would help if you described the real problem you are trying to solve a bit more. For example if you have many readers and one writer, there are other approaches.
What do you mean by "concurrency"? Do you actually mean "multiple readers/multiple writers", or can you get by with "multiple readers/one writer with limited latency"?
I intend on writing a small download manager in C++ that supports resuming (and multiple connections per download).
From the info I gathered so far, when sending the http request I need to add a header field with a key of "Range" and the value "bytes=startoff-endoff". Then the server returns a http response with the data between those offsets.
So roughly what I have in mind is to split the file to the number of allowed connections per file and send a http request per splitted part with the appropriate "Range". So if I have a 4mb file and 4 allowed connections, I'd split the file to 4 and have 4 http requests going, each with the appropriate "Range" field. Implementing the resume feature would involve remembering which offsets are already downloaded and simply not request those.
Is this the right way to do this?
What if the web server doesn't support resuming? (my guess is it will ignore the "Range" and just send the entire file)
When sending the http requests, should I specify in the range the entire splitted size? Or maybe ask smaller pieces, say 1024k per request?
When reading the data, should I write it immediately to the file or do some kind of buffering? I guess it could be wasteful to write small chunks.
Should I use a memory mapped file? If I remember correctly, it's recommended for frequent reads rather than writes (I could be wrong). Is it memory wise? What if I have several downloads simultaneously?
If I'm not using a memory mapped file, should I open the file per allowed connection? Or when needing to write to the file simply seek? (if I did use a memory mapped file this would be really easy, since I could simply have several pointers).
Note: I'll probably be using Qt, but this is a general question so I left code out of it.
Regarding the request/response:
for a Range-d request, you could get three different responses:
206 Partial Content - resuming supported and possible; check Content-Range header for size/range of response
200 OK - byte ranges ("resuming") not supported, whole resource ("file") follows
416 Requested Range Not Satisfiable - incorrect range (past EOF etc.)
Content-Range usu. looks like this: Content-Range: bytes 21010-47000/47022, that is bytes start-end/total.
Check the HTTP spec for details, esp. sections 14.5, 14.16 and 14.35
I am not an expert on C++, however, I had once done a .net application which needed similar functionality (download scheduling, resume support, prioritizing downloads)
i used microsoft bits (Background Intelligent Transfer Service) component - which has been developed in c. windows update uses BITS too. I went for this solution because I don't think I am a good enough a programmer to write something of this level myself ;-)
Although I am not sure if you can get the code of BITS - I do think you should just have a look at its documentation which might help you understand how they implemented it, the architecture, interfaces, etc.
Here it is - http://msdn.microsoft.com/en-us/library/aa362708(VS.85).aspx
I can't answer all your questions, but here is my take on two of them.
Chunk size
There are two things you should consider about chunk size:
The smaller they are the more overhead you get form sending the HTTP request.
With larger chunks you run the risk of re-downloading the same data twice, if one download fails.
I'd recommend you go with smaller chunks of data. You'll have to do some test to see what size is best for your purpose though.
In memory vs. files
You should write the data chunks to in memory buffer, and then when it is full write it to the disk. If you are going to download large files, it can be troublesome for your users, if they run out of RAM. If I remember correctly the IIS stores requests smaller than 256kb in memory, anything larger will be written to the disk, you may want to consider a simmilar approach.
Besides keeping track of what were the offsets marking the beginning of your segments and each segment length (unless you want to compute that upon resume, which would involve sort the offset list and calculate the distance between two of them) you will want to check the Accept-Ranges header of the HTTP response sent by the server to make sure it supports the usage of the Range header. The best way to specify the range is "Range: bytes=START_BYTE-END_BYTE" and the range you request includes both START_BYTE and byte END_BYTE, thus consisting of (END_BYTE-START_BYTE)+1 bytes.
Requesting micro chunks is something I'd advise against as you might be blacklisted by a firewall rule to block HTTP flood. In general, I'd suggest you don't make chunks smaller than 1MB and don't make more than 10 chunks.
Depending on what control you plan to have on your download, if you've got socket-level control you can consider writing only once every 32K at least, or writing data asynchronously.
I couldn't comment on the MMF idea, but if the downloaded file is large that's not going to be a good idea as you'll eat up a lot of RAM and eventually even cause the system to swap, which is not efficient.
About handling the chunks, you could just create several files - one per segment, optionally preallocate the disk space filling up the file with as many \x00 as the size of the chunk (preallocating might save you sometime while you write during the download, but will make starting the download slower), and then finally just write all of the chunks sequentially into the final file.
One thing you should beware of is that several servers have a max. concurrent connections limit, and you don't get to know it in advance, so you should be prepared to handle http errors/timeouts and to change the size of the chunks or to create a queue of the chunks in case you created more chunks than max. connections.
Not really an answer to the original questions, but another thing worth mentioning is that a resumable downloader should also check the last modified date on a resource before trying to grab the next chunk of something that may have changed.
It seems to me you would want to limit the size per download chunk. Large chunks could force you to repeat download of data if the connection aborted close to the end of the data part. Specially an issue with slower connections.
for the pause resume support look at this simple example
Simple download manager in Qt with puase/ resume support